I'm a completionist - also i care about how my character looks.
It's not that witout cosmetics or xmog i won't be playing and yes, they don't add basically anything gameplay-wise apart the conveninece of automatic gold looting.
I farm for them because i want them all - same with achievements and all that stuff.
Rewards are cosmetics because as soon they put a +1 mainstat ANYWHERE, everyone would scream at the "don't force me do things i don't like".
As for effort/impact on the game, models and textures this small require a very low commitment, and in game performance is not affected in ANY way. If anything, they should solve the dot issue which is making the game lag as hell.