Its not one-sided, you are just not presenting your argument well at all. As a fantasy set game, your immersion will always be altered slightly to not make sense with reality. It is a good thing however that I don't depend on the immersion of reality to satisfy my needs for it. I play a fantasy based game, which means I want items, situations, and worlds that are far out of reality from my own.
The argument of vanity slots comes down to a technical scale if anything. Some believe in making a unique looking character to help them immerse themselves into the world. Others like to wear the latest gear to display what they have achieved to everyone. There really isn't a right or wrong if vanity slots are good or not, because both sides can make a valid argument.
However, what I do believe is that we should be given a choice to use this standard feature. Of course you have to make restrictions such as you can only wear what your job can, no event items, and so on.
There is no victor in this argument because no one is wrong, it is an opinion based matter. In short, use it if you like it, or not, up to you, and it shouldn't affect your experience with the game.
EDIT: Also like to point out that seasonal items like bathing suits and reindeer suit won't work with vanity well even if you could put them in there. Bathing suits are underwear and whatever you are wearing over it would cover it, vanity or not. Reindeer suit also conflicts with a few pieces unless you are wearing gear with no feet.