I think the idea is pretty much summed up in an exchange I overheard at a game a while ago;
"Ohh, and we don't play with alignment. Sometimes it's too hard to figure out who would be what."
"How come? What's so hard to understand about playing Lawful Good?"
I'd venture to say that people would have the same taste for classes and alignment, some seeing them as useful guides, and others seeing them as pigeonholes for your character. And while there is a bit of fluff in both the DMG and PHB about customizing classes, it doesn't give you any guidelines or templates to go on. I think those would quell a lot of the classless fervor, at least for a while.
I will give classless the following, though. It allows for easier creation of the true oddballs (even if most of the results are either skill mishmoshes or virtually indistingishable from classes), and a points => skills system is a lot better at showing character growth than classes are (both because of the sudden power jumps, and the unlikelihood of getting those extra skills/proficiencies/what have you when you really need them storywise... would you be unable to learn a new language because you levelled up a sesson or two before being washed up on some strange shore?) But customizability is the keystone of freeform, as opposed to the easier time working with and balancing static packages. YMMV.
Of course, classes are much better from a game design standpoint. But freeform/classless is better from a story standpoint, provided that you and your players can handle the lack of structure. Your play style and opinion of other gamers should show your preferred style.
(Although as for people who think that class-race-alignment should explain them in full detail, I'm allowed to wonder.)