There is nothing wrong with collecting things. We all know there is a collection of just about everything known to man. And collecting can be a worth while and rewarding hobby.
IMO, a collection should consist of special/limited edition items. They will hold more monitary value in the future, as well have a greater resale value once they are out of production. I realize not everyone collects for that reason, and do so more for sentimental value or pure enjoyment.
I think alot collectors do so knowing that one day that piece of makeup will be vintage and will hold a sort of historical value for future makeup lovers. Like makeup items from the early 1900's for example. We just marvel over those keepsakes and an unopened/untouched item from the 40's is highly desired by vintage collectors. Maybe I am reading to much into this whole collection thing lol.
Again, this is solely my opinion and I don't think my opinion should be everyones. If you enjoy collecting makeup, then that is awesome. Don't let anyone try to discourage you from enjoying what you enjoy.
I try to keep my own "collection" of makeup on a smallish scale. I do have more than most all my friends, and people I know, but I also do makeup for them and others so I have to have more makeup then I personally need.
I don't really collect makeup as a hobby personally. I collect owls. lol If it has an owl on it I want it. I am the crazy owl lady. I have keychain, wallet, jewlery, little home decor and lawn ornaments. It isn't overtaking my house, but pretty much every room has at least one owl in it. lol.