There are much more expensive hobbies for sure. At least this is one that you get daily use out of most of your purchases.
My wife puts up with my expensive hobbies pretty well, all bills are paid and we aren't struggling. It's extra money, so why not have a little fun with it? I rarely say no to anything she wants to buy so it's not like I have all the cool toys etc. She recently has gotten into hand lettering, calligraphy, and a few other hobbies on her own that I've bought expensive supplies without question. For a while when I was buying lots of pricey Coral/Fish for my reef tank, I was giving her the same $$ in cash for her little slush fund that I was spending on my stuff. That was a good way to keep my own spending in check.
As far as getting several watches? YES! And the easiest way I've found to make her not worry about the cost is telling her about the M2M sales on here. As long as you keep your watches in decent condition, you should be able to recoup 70%+? of your initial investment should you need/want to sell. Of course if you beat or mod the crap out of them and nobody else wants what you've done you won't get it back. But for the most part it seems like we should be able to sell that watch you paid $400 including shipping for $300 easy in good shape. Not very many other hobbies that you don't lose value on.