I don't think of Ki as another form of energy. I feel that Ki is when the body and mind are in sync, through proper posture and breathing. The purpose of Ki exercise is to devolop Mushin. Or to state it another way when you are in Mushin you are extending Ki. I don't give Ki to much credit as a super energy source that will cure the ill or give you super powers.
My personal definition of Ki exercise is to get to a state of mind. Have you ever had the experience of having the attacker seemingly moving in slow motion and you have a lot of time to set up the perfect response? I think that is the most common experience, and by training in Ki you are able to get that ability frequently.