I agree. Luke Cage has, more or less, always worn street clothes (or something similar to that effect), so if he were to suddenly wear a uniform of sorts, I don't think it'd look... Well, it'd certainly take getting used to!
Some characters, like you said, look better in their costumes or a variant of it, IMHO. Like Gambit and his trenchcoat. If he didn't have that, it wouldn't look like Gambit to me -- just some Joe Schmoe wearing the distinctive red-on-black contacts. For some characters, their costume is the only thing that distinguishes them from the next.
I remember reading the Generation X comics back in the 90s and being confused when Husk and Emma Frost were in the same scene together and being almost unable to tell them apart (this was when Emma wore her more "conservative" outfits, but the fact that they were white in color helped out).
This sort of confusion also happened to me recently with USM. I could barely tell Gwen Stacy and non-uniformed-Sue Storm apart!