QUOTE: Their reason for being is, again, different than that of the Avengers.

Not really.  They both exist to fight bad guys.

I guess I read these things differently than everyone else. I thought the question was, essentially, "why did you give us brightly-colored costumes instead of keeping us in the old school uniforms?" Which does make a certain sense. The X-Men aren't the Avengers, after all. 

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If Sue Storm redesigned the FF uniforms so that they were all wearing unique costumes, they would still be "superheroes," so there'd be no sense in Ben asking, "Why'd you make us dress like superheroes?"

The professional athletes allusion that JB made is spot-on -- not just in how superheroes dress but in how the world relates to them. The existence of professional athletes has not resulted in a drastically different world, much like the existence of superheroes does not result in an unrecognizable world for us.

Slight digression: I've always viewed the DC Universe as being the comic-book equivalent to Los Angeles. It is more "artificial" and the "heroes" (celebrities) are overt and distinctly different from us. Marvel, to me, is like New York -- grittier, a little more down-to-earth and "real." Just as I might see Robert De Niro browsing in the same record store I'm in, a Marvel Universe resident might see Spider-Man crawling up the wall behind him. He then goes about his business because that's the world in which he lives.

A couple of posters have already mentioned the names of a creator (or two) who suffer from the mindset of "kewl." As for those military-styled costume "upgrades":

For me, a project like JMS Supreme Power, was inferior  to the old Squadron Supreme maxi-series by the late Mark Gruenwald - - - specifically because of this attitude.

From the moment, those designs showed up on my desk - - - had I been editor - - - I would either have killed the project outright (thinking that the potential creative team held the developed property in utter contempt) or redirected their efforts into a new title; using what was presented as something entirely new (without tarnishing my established property).