First, I'm going to say this. I am a tournament player, and competitive player.
I say, Final Smashes should be in, but, on a very low level. Why? Well, yes, They are a single powerful move that can give a huge advantage, or kill people (sometimes instantly, but most likely they could be edge guarded which'll kill them anyway). However, this is assuming they hit.
A final smash, in all honesty is comparable to a lot of techniques in melee. For example, Wavedash. It is helpful, yes, and some characters have better ones than others, such as Bowser's to Marth's. It is not a win all technique as people claim it to be, but it is a situational advantage that can be implemented in different situations to give you just the edge you need.
Also, Mind games anyone? Correct me if I'm wrong, but if you're playing someone and you get a smash ball, you're going to be on edge, because if they hit you with it, it's gonna hurt (or, they're gonna get healed, in peach's case). It adds another level of mind games, that open up several options when fighting your opponent. This is another huge comparison relative to that of wave dashing.
Yes, I can see someone saying "Well you can't pull Final Smashes out at will like you can Wave dashing." Yeah, you're right. But on the flip side, if you're saying that you can't pull out Final Smashes that much, then what's the problem with having them in the competitive scene? You'll get 0 - 2 final smashes per match, MAYBE. So are you saying you can't dodge ONE? They are all stupid easy to dodge, and hell, some of them could even be shielded for the brute of it. Has anyone taken that into consideration? Yeah, the Zero Laser would hurt and all, but if you shield it from the middle of it you'll get hit for maybe, 10%? That's less than a fully charged neutral B from samus.
All in all, people who can't open up to Final Smashes being just another small part of gameplay, need to stfu and either adapt, or QP. It's a new game, so may god strike us all down for having a new option that all characters have.
This is almost as bad as wave dashing. It's in the game, all characters can do it, so get over it.