That's not nearly as interesting though. If they were done as completely new characters, people would complain that they're just rip-offs. Characters like Ultimate Cap, Ultimate Thor, Ultimate Spider-Man, and Ultimate Fury all work better as fresh takes on existing characters rather than as completely new ones.
I love the Ultimate line. I prefer the more realistic approach to superheroes to the 616 universe's overall cheesiness (though thankfully, the 616 books have become more realistic in the last few years). I also like the fact that the Ultimate universe is basically a playground for my favorite characters where the stories actually matter; you don't have to worry about things being reversed or retconned down the line. It's pure entertainment, and the fact that it doesn't have to worry about sustaining itself indefinitely helps add gravitas to books like USM and Millar's Ultimates--though those are the only Ultimate books I really care about.