I think that while they are nice to have, you shouldn't have to be railroaded into them. They should have allowed you to kill that Walker any which way you choose, but if in the event you perform the button combination to trigger the cutscene, then that happens instead. It'd be a nice surprise for gamers to find badass ways to take out the boss a little quicker, but not be forced into them.
I don't think that's possible by todays (or at least last gens) standards, as you'd need a different cutscene depending on what your finishing move is AND you'd need never need a QTE as you can just keep hammering a button until life becomes 0.
But I would like to see that as well, it gets kind of annoying having a challenging boss fight until the QTE hits you and you die, just to breeze through it the second time because you know what button is going to come up.