To be honest, I've had a lot of good results when tweaking color after scanning (though sometimes iffy results, too)... but then I've never had a basic complaint, either, along the lines of "ugh! the true colors didn't come out in the scan!" (or "it ate the colors" or whatever; sometimes I'm not even sure what that means). That's why I was saying that for the variety of things I've tried to scan, I've found even the basic, non-tweaked-before-scanning settings to work fine on the scanner I have.
But another thing I've always wondered about folks who don't like the colors they get from a scan, or who don't play with the colors afterwards, is what resolution they're scanning on. To tweak afterwards, my experience is that you have to scan on the highest resolution you can. I never scan color at less than 200 dpi, more often at 300 dpi, and if I'm especially concerned, at 600 dpi -- even for pieces I intend for web posting in the end (which for me is all of them). Scan high, tweak at high res, then at the very very end when you're happy, re-save at 72 dpi for web posting. Scanning at low resolution just winds up with less information captured, so of course there isn't much to tweak to make better.
(Either that or, I suspect, those folks don't have access to much in the way of programs in which to do the tweaking afterwards, I don't know.)