You can't argue that FF, or even Square Enix in general, has become a trend. But there IS a reason for that: They can put out high quality games. Many will agree that their best games were made 10-15 years ago, and that's why Dissidia appeals to a lot of people. It's really a nostalgic thing. Not only that, but improvements and added content have been added to Square's favorite and most iconic characters, and most fans will buy it simply for that reason.
I think Tofu put it perfectly...
The storylines and graphics have become so paramount that the gameplay is haphazard; just something to bore us between cutscenes.
RPGs in general have become like this. Fortunately, the Final Fantasy series is known for coming out with something new with regards to the gameplay with every game. It's not just the story that's different. Now they're focusing their newest "RPG" gameplay on the fighting, perhaps for lack of story. Whether this works is up to the consumer. With a minor plotline, the gameplay better not bore us. Critics' reviews and the fanbase's reviews are always going to be polar opposites. Look at Transformers 2. Critics gave it less than staller scores while the fans thought it was rated E for EPIC!