Personally I liked the entire series until they changed the battle style for FFXII though to be honest... it still was a good game, but I never really went past that. Of course, I'm still rocking the PS2 :]. I have seen FFXIII art and always thought it looked cool. Hopefully they did better with XIV.
The last one I played was 12, but couldn't get far enough into it to beat it. The only reason I got as far as I did was because I was one of the few that found the battle style to be a great improvement (at least in that, "You already know you are grinding so the less button presses you have to do the better" sense.)
I didn't really appreciate the setting at all until I finally realized how entirely similar it all is to the setting/characters of Firefly. The story of the game itself gives me absolutely nothing, but It's bearable if I mute the cutscenes and pretend I'm watching Firefly instead.
Yeah, do a youtube search on it, you'll see exactly what I mean.
You ask why with FFXIII, but anyone who is even remotely aware of Final Fantasy X had to have seen it coming. At least anyone with enough of a brain to pick up on patterns of crappy games. FFXII pretty much confirmed it for me, which makes me rather ashamed to have bothered even playing that one in the first place.
The fact that people are still looking forward to XIV is proof positive that square could keep getting away with crap like this all the way up to FFXX even.