Over at the Japanese SFIV blog, Shiozawa-san recently shared some of concept sketches for the SFIV alt. costumes.  In addition to the art, she explains that the motivation behind the alt costumes was that they be outfits the characters would actually wear in the course of their individual pursuits. 

So for instance, Blanka is adventuring through the jungle, Cammy has a "cold weather operation" suit (although I'd suggest, I dunno, pants?), and Dan has a special training outfit for all the extra time he spends practicing to actually win a match.