You're absolutely Right. These are larger than like extraordinary fictional characters. People shouldn't be so concerned with realism when a man can Fly or run at super speed. If you are then you're missing the whole freaking point. Superheroes wear bright colored uniforms and stand out from the crowed, that's why they're SUPER!
You're also very Correct. The X-Men costumes could have been easily adapted for the big screen without putting them all in the latest Biker Attire. I felt like they should have all been riding Harleys or something.
The Thing to remember with these Superhero teams is that most of them are INDIVIDUALS with vastly different personalities and super abilities. A HUGE part of their Dynamic and Charm is that EACH character is different and special in their own way! Their attire reflects this! The X-men very well could have been done in varying colors, like yellow and blue. Last time I checked they ALL rocked something very different in color and design.
I Blame singer for the whole dark leather X-Men and bland Superman costume fiasco, but don't get me started on that atrocity.