I think if they could do it, it'd be an all or nothing deal - not sure they'd be able to do it so your character has glams but everyone else doesn't as I assume a glam is a sort of "flag" in the game code "Show glamoured item: 1" or something like that (I could be wrong, I'm no game programmer, just going by past experience with other programming/configurations I've seen).

Also what is the main reason behind needing this? Is it people wearing costumes in group content? Is it the costumes people wear in the city areas? Is it someone harassing you in a subligar?

I'm not dissing the option or idea - just genuinely curious why you actually want it? Personally I love seeing other people's glams and have lost count the number of times I crack up running group content (Prae or Castrum) with a bunch of metallic animals. However, I appreciate some may hate this for immersion qualities and that's fine.

The option described would help with some of these (i.e. costumes in group content), but not fix them all. As an example - say you keep getting followed around by a roe in a bright green subligar any time you go to Limsa Lewd-minsa. You click the option to turn off the glamour . . but he stays the same. Why? Because he hasn't actually glamoured an item . . he's actually wearing it. In areas where there's no risk of getting agro from mobs, walking around in level 1 items has no draw-backs.