I think that any current generation fighting game needs to have easily accessible multiplayer functions. Street Fighter IV and Tekken 6 are set up so that you can find competition without actively having to gather players from your area or from an online community. It looks to me like Dissidia Final Fantasy lacks this, but this might be the fault of the Playstation Portable rather than Square Enix.
I agree that competing against the computer is alright, but, when it comes to fighting games, I feel differently because these games really shine when you are playing with another human rather than an artificial intelligence with poor strategies or design.