In the words of Jay Wiseman from a seminar that I attended over the weekend: good pain, good; bad pain, bad. A submissive will know the difference (i.e. during bondage). Personally, I have a lot of vaginal/urethral infections, and I sometimes use that pain during intercourse to help me "pain train" a little that does not necessarily have to be SM related. Other than that, non-SM pain can be used to condition one's body (see martial arts practices), but your brain may start to get fuzzy if the good and bad pains start to mix.