My guess is that smash balls were turned on maximum frequency in the demo, so people could see/use them. So that's probably the most you'll see of them in a match. It'd be stupid, I think, for them to appear that frequently normally.
Also an all FS mode would be kinda cool. At least at first. After a while, though, you would probably get bored of just using everyone's final smash over and over. It would just turn into a race to see who could get to each smash ball first.