What, if any, danger lies in a dirty microwave?

Several times a year I find myself cleaning up after renters who are mainly college age children. Usually, one of the nastiest jobs is the microwave which will invariably have the remains of several meals splattered all over them; sometimes I swear I was probably the last one to clean it a year previously.
So, my question is: Does all the left over foodstuff stuck to the walls pose any health risk to a person who then uses the microwave on another meal or does the microwave process kill all the microbes and bacteria growing in the old food each time it is turned on and is it only aesthetically displeasing to view whatever you have been eating over the last month? At least to some of us.