Kind of defeats the whole point of a series, don't you think?
Hell, for the most part, I think they keep calling most of the game's Final Fantasy because of the difficulties in acquiring a trademark for alternative titles... That and the added publicity/devoted fan base/money of just slapping on the same ubiquitous title.
I honestly think there are at least 4 different "series" in FF (Not going to make the "Not so Final" Joke).
Bastardization of a series FTW.