I can confidently say it's an extremely difficult task to implement NPC's that "learn" based on a players behavior. In your case with Street Fighter (Tulio C), indeed it WAS alot of work, but when you examine the outcome.... it's was good. It would've enhanced the entire experience for ANY player. Now, the question that begs to be answered (or even asked) is whether it's beneficial to exert as much effort into simulating credible AI in ANY game nowadays? Is it? Just [another] topic to discuss.

I think game designers want control in the ai process for simulating human style behaviors such a sarcasm, etc. Now, if you had an ai who's properties could be preset and it would not veer off from them then that would be nice to have. Then there is the whole issue of how to tie this into the mesh model, animations and all that, especially for physically different models. But I agree that simply giving the illusion of good ai is enough. I personally like to see each character to have its own unique emotions. So some chars are comedians, some are serious, some are scared, others are preoccupied, etc. I think the ai would have to be evolved to the point that all these things would be possible and it might turn out that an easier human controlled system is more desireable.