Breaking the formula a little bit, but:
Just because we aren't the SCA doesn't mean you don't need garb.
Whenever a serious conversation about garb standards starts, someone invariably says, 'well, we aren't the SCA!'* Of course we aren't, we're Dagorhir. And Dagorhir requires garb. Telling people who call you out for breaking the rules by wearing a tshirt onto the field that they should go join the SCA doesn't change the fact that Dagorhir's rules have a minimum dress code.
And the SCA isn't a history geek paradise - it's a bunch of nerds, some of whom like history, some of whom like to dress up in pickle barrel armor. That is, they're just like us, only bigger, a little richer, and generally more pseudo-historical. If you want to use 'Quit Dagorhir, go play ______ instead' argument, tell people to join a living history group. They're the people who make everyone wear real historical stuff. The SCA doesn't.
* (This rant isn't aimed at your recent comments, Athelas; I know you weren't using the SCA thing to justify illegal stuff. Others do though, and this rant is aimed at them.)