I believe that developing one's character, self-defence (and defence of others), and personal challenge are all very valid reasons to train. It is just that I never understood why some (particularly in the AIkido community) choose to separate them out. Why not incorporate all three into one's training? Without moral and character development one risks being no more than a trained thug and on the other end of the spectrum, one risks being a self-righteous prude who is largely helpless to help others in a physical situation despite having trained in a martial art.
I certainly cannot understand why someone would want to have a self-defence situation just so they had a "opportunity" to use their stuff per say, but it seems wise to be ready in case and if the "opportunity" never comes its OK. I mean, one is still a better person for training, conquering their demons and others (even if they didn't realize it, they will have been safer when you were around).
That's pretty much my opinion. I think all these aspects of training have to work together to make for a fruitful training experience. Everything needs ballance.
You can't be so far delved in the art that you forget the martial part, nor can you be so far into the martial effectiveness that you forget that its an art. These techniques should be martially effective, but they shouldn't be learned with the hope of using it some day. I believe in refining your character, through the refinement of martially effect technique. It needs each other in my opinion.