The real problem is that loot boxes are gambling, and are addictive, period. It triggers something in otherwise rational people, especially children, that makes it feel good to spend money on mostly non-gameplay-altering cosmetics.
As a medic in team fortress 2, I usually pocket and ubercharge the players with cosmetics. It's an important team resource, and the odds of someone with a "default" skin making good use of it are fairly low. Much like plumage for a bird it's a reliable signal of "virility" - if you're invested enough in the game to own a $200 hat and a coordinated outfit then you've probably played it long enough for me to trust you sight-unseen.
http://i49.photobucket.com/albums/f278/katietiedrich/comic26...
If you're a pro player with the default skin, you're gonna have a bad fucking time, because you're last priority for heals, you're not getting an ubercharge, etc. That is gameplay-affecting even if the cosmetic itself is not.
And of course it's a vanity component (at least for me, though other posters have raised other factors). Everyone's different with different preferences and as long as there's not a problematic spending aspect I think it's fine...but this links back to the gambling issue. Personally I only buy cosmetics in a few games I play a lot and that don't use loot box mechanics.
It seems to me that it definitely has a psychological impact for some folks (myself included) in both off-line and on-line competition.