I prefer offline play for Fifa because:
-I'm not very good at it, and would get destroyed online
-Soccer/football is not my #1 game. I simply don't have as much time to devote to it to make myself better. In order to play online, you better practice a lot, and I don't have the time. I play online for NHL, and that's it. Even then, I stick to franchise offline most of the time.
-People online are... not always conductive to a perfect gaming experience. I only have 1-2 hours to play most nights (if that) and I don't need some teenager screaming in my ear for that time. Also there are so many cheesers and idiots ruining every game out there.
-I don't always want to play ManU, Barca or whatever other top teams everyone uses online every time.
-AI in sports games should not be dismissed as irrelevant. Lots of people play offline. Every AI cheats... they have to or we'd all figure them out. Still, you can get good single player sports games. PES 2008 was fun offline (far from perfect, but it gave you a good offline game). The PES 2009 demo feels like the AI is much improved (we don't have the final version here). The backline is much smarter and they defend the wings better.