I agree with Wolf Kanno. S-E will make lousy spinoff games for no reason other than generating sales, because they know the obsessed fanboys/fangirls will buy them no matter how awful they are. To me, spinoffs go far beyond 'tarnishing the reputation' of their original titles, they actually make me less interested in the original titles as well.
As for FFVII, which I have made very clear that I feel that it is extremely overrated, a mediocre game at best, all the spinoffs have done nothing but make me hate the VII series in general. Don't get me wrong, I don't hate the original VII; I just think it's awfully overrated, and has generated an irritating fanbase who are starry-eyed over lousy characters and Sephiroth, who in my opinion is one of the worst villains in the Playstation era of Final Fantasy. Not only that, they are blind to the fact that the story was confusing and rather dull.
FFX-2 did the same thing for me. I enjoyed FFX a lot, and I had very high expectations for the sequel. What did I end up playing? A lousy spinoff that ruins the main story, soaked in half-assed fanservice. I have had no desire to replay FFX since. Think of it as 'dislike by association'.
I'm even skeptical of the FFIV sequel. FFIV is by far my favorite Final Fantasy overall, and something tells me the sequel will do nothing for the story.