Only in the most extreme cases, like a World War II, would a draft be needed (or helpful).
The draft in the 1960s brought in a wave of malcontents who poisoned the Army. Read "American soldier" by Tommy Franks, who details his struggles in dealing with dopers and punks who were in the Army as a result of the draft and were finishing their duty after Vietnam. They hurt the whole organization. Drafted soldiers are simply inferior to volunteers.
I read that. Opened my eyes to why we shouldn't have a draft now. To many losers would be brought in.