I would agree that FD is the most balanced stage if it weren't for one factor: chainthrowing. Play Savath on FD with a fastfaller, and tell me if you still think it's fair. Perhaps I'm slightly biased, being a Falcon player, but our objective is to be as fair as possible, unless I'm mistaken. If forced to pick only one stage, I'd sooner choose PS than FD.
I for one see no problem with using the standard neutral stages. It's old news that some of these stages are not completely "neutral," but they all balance out when used in tandem. That is to say, Yoshi's Story favors Marth and Dream Land favors Peach, but not to such an extent as to completely throw everything off. That's the whole point of counterpicking, really. Knowing what stages are good for your character and how to deal with certain stages is just as important as knowing your character and their matchups.
Sorry to rant like that, but what I'm basically trying to say is that I don't think FD only is a good idea. It's not that I question the fairness of the stage. I just think people should be rewarded for counterpicking intelligent and truly knowing every facet of their character. Moreover, I'd think some would find playing nothing but FD for 8 hours boring.