I think most answers in this thread are almost responding to a version of the question posed by the original poster, which is to say this is 'why I buy both Gens and Reps, not just one or the other'. I think this is key, reps don't offer you the variety and flexibility that gens do, but gens don't offer you the variety and flexibility that reps do.
TLDR; I think both are a valuable part of any collection. If I were to expand.
What reps do not offer:
1. High end, high complication watches / independents: You can't yet get (unless there is a whole segment I don't know about) high end complications and independents. If you want a Moser Perpetual Calendar, a Lange Triple split or Zeitwerk, FPJ resonance, a PP double split or minute repeater, MB&F double chrono or Perpetual Evo, Rollie Annual calendar... you just aren't getting them in the rep world. This is not something many are concerned about admittedly, but it is still a gap which won't be closed.
2. Sufficient Design variety: There isn't the range available to a purchaser in the rep world across price points. From low to medium to high end, mens, women's and children's watches. In the gen world there is just far more variety which fits all tastes, size and price-points. This is not the case in the rep world - majority (not all, of course) is Rolex, PP, AP. Which is fine, that satisfies most tastes I think but maybe not for a collector over the long term.
3. Material variety: Money no object you could wear a carbon-fibre 70g Hublot on the wrist one day (please don't - save yourself), followed by a platinum Daytona or white gold FPJ the next. You could wander around London, or NYC and Singapore on a weekend and have solid gold, silver, titanium, carbon, tantalum etc. on the wrist to enjoy. At high and low budgets you can have an enviable range of materials on your wrist.
4. Quality: Ish - if it is a mass produced Rolex etc. frankly a good stainless steel copy of a 116500, 124060, 126610LN etc. will get you 85-90% of the gen quality but that is for a few models and even with those there is always that 10-15% which is off - that extra accuracy, the likely longevity, that extra polish with finishing. That being said frankly AP and PP have been sitting on their hands with their entry-level models and the movements have not evolved for years, and years, and years. Go into an AP boutique and try on a 39 and change the date, ask about WR and note the power reserve and be very very confused.
That PP introduced the 6700G range as the 'modern' base Calatrava with an upgraded movement but it still has only 35-45 hours power reserve is comical when compared to the H.Moser & Cie base 3-hander movement or any Tudor BB ( I know its dress not sports, but its still 3-hander / 3 hander with date comp).
5. Niche, watches with history: Want a watch that has a story, that was on the wrist of a soldier, sailer or candlestick maker for thirty years. Want a watch only issued to military unit because it was your family unit, a dive school that your aunt and uncle attended, a graduating class of jet pilots that your best mate graduated from who since bought it, a certain employer. Nope. Not only do those not see the light of day for the most part - there isn't the financial incentive to reproduce.
6. Asset / Investment: There is more potential of upside if you buy right with gens but reps are as liquid when compared to a varied gen collection, just the values more likely lower. The most liquid gens will take approx 5 days from the point you would like to sell to having £ in the bank if you use a scale watch dealer (exceptions apply). Factor in insurance, servicing, buying the models you thought you wanted but don't, locking up cash and it's a more interesting story as a scale watch collector when you work across brands. Parking cash in a small physical asset that isn't going to materially depreciate is also valuable to some which is an often overlooked benefit too.
What reps do offer:
1. Accessibility (£ $): No two ways about it, medium-end watches are generally speaking overpriced, high-end almost always are, ultra-high end are crippling. A lot of the 'goal/grail/hype/cool' watches are far out of the range of most financially. Make no bones about it, in this authors opinion people should get a replica rather than put themselves in a precarious financial position and load themselves up with debt or financing. An 18 year old who buys a replica today, could buy a PP grand complication in 15 years having got that first foothold on this forum - buying when they can afford but enjoying the hobby for years before.
Look at JLC and the comical increase in pricing multiple times in the past 18 months, Lange and the 20-30% increase across the board the other month, Tudor (an accessible brand) and the 10% + increases staring to compound and raise eyebrows. Not great.
2. Accessibility (access): Can't get the damn things when you want, usually. Self explanatory - people know the Rolex challenges. Which to be honest, like others have mentioned vary across regions and stores and it's almost (is) unfair how easy these actually are for people connected properly who aren't always genuine collectors. For PP, Lange you will have to snake your purchases through the collection categories rather than go for what you want when you want if you want several (even if you are a noted collector). For MB&F, FPJ - better make your way into the F&F to get what you want, the lists go on. Even when you drop to the more accessible but hype like some from Baltic, Studio UnderDog, Christopher Ward, Tissot you can wait months. I've personally waited longer for some entry-level hype watches than 'in-demand' professionals / sports.
3. Try before you buy: Not too much more to say - try before you buy and I do this a lot. Not sure if you want a Submariner or Submariner Date then try them both delivered to your house in days. Don't like, sell them. Yes you can go into stores but that doesn't always work well for everyone - i) stores don't always have stock and when they do you won't spent much time with these watches ii) many are intimidated in these high end watch stores and are treated badly - 'don't have an appointment, bugger off please', iii) plenty of people aren't located close to watch stores. People getting pied off in stores is something I see so often and I don't like it, everyone has to start somewhere and not all people are as comfortable, have large spends and know the people who work in stores. Go to watch clubs you say... well some of these and the people in them - that's a whole different post.
4. Insurance, Servicing, Ancillary costs: These go up or down with scale but it is not nothing. Insurance can be low - often sitting at c.1% of watch value for something like a Sub is good, but this adds up. Independent servicing (not all - some are VERY good in this regard) or complication servicing takes time, £/$ and pain sometimes. Paying for safe deposit boxes, serviced or monitored alarms costs £ too.
Pay to play: Even top tier collectors for the v. high end brands will be required to buy watches you don't really want on the journey - or watches you want but too frequently (ironically). It's the price of entry and doesn't matter who you are. Can you get rid yes, but for the v.high end it's a small world and whilst you might get a shot across the bow on the first one you dump v.v. quickly as everyone knows it was a. stepping stone, twice and they will sink your battleship.
5. Loss: Lose a replica. Ok, don't care .. what's for dinner. Lose a gen, oh this is going to be fun - police, insurance, watch shops and registers etc.
6. Security: If we are talking well-known watches like sports PP, sports Rolex etc. then one has less concern in many locations and times with a replica. Obviously you don't want to flash either around in certain locations or at certain times but this is somewhat self explanatory. Also with reps you don't need to make sure you strictly adhering to insurance conditions / expectations ('oh did I put it in the safe' etc.). Side point, with reps your most frequent trip isn't to the bank and WAITING FOREVER every time for the box opening procedures.
Im sure there are more reasons and arguments both sides, just adding in two cents.
