I think its best to train with both trained and untrained fighters. I don't think anyone is fully prepared to deal with a mugger, someone high on drugs or drunk. Thing is in a real situation your bound to be outnumbered and attacked either from behind or by surprise and attackers sometimes do carry or will use something in the vicinity as a weapon. There is also the threat of spending jail time just for defending yourself.
I know I am prepared to deal with someone high on drugs and someone drunk, because I have done it! I have had two self defence situations the first on was a drunk guy (two but I had a friend), the second was with a guy who had been sniffing glue and paint (don't know if this is a stimulant or depressant). I don't think I will every need to deal with a mugger as they target easy weak people. I do, however, feel confident from my two encounters and training that I could defend my self against an untrained person. If I did not I would not train at a self defence school.