The ol' disability = purity equation doesn't make much sense to me, either. I think it may be related to the belief that a weakness in one area is compensated by a strength in another -- that blind folks have super-human hearing, and the such like. If you believe that the body and the spirit are seperate from each other, you might conclude that a "weakness" in the body leads to a strength in spirit, or something.
And as for having the qualifications to be an angel... I thank you. But I'll decline that job offer. I have enough trouble being human... ;-)