If the class options are well balanced, and the swaps fair, I expect people would use the options like Lego bricks.I think we would end up with three, maybe four de facto classes: the optimized fighter/tank, the optimized ranged DPS, the optimized rogue, and maybe the optimized buffer.
Casual players will probably just use the readymades. But enough players need customization.
Consider how popular feats are because of their customization.
Of course, if most of the options are crap, and only three options are good, then obviously the optimizers and many others will just pick the good options.
Why make Fireball the only good option and then make Flamestrike crap? That is naive design that will end up with players refusing to eat crap.