I think military service experience can be beneficial for a commander in chief, but I do not think its mandatory or preferential. In the case of McCain, I felt that he looked at everything militarily and would have made the choice for military action before anything else, he still jumps the gun and salivates at the thought of war.
Not everyone is cut out for it.
I don't think that McCain would have looked at military action as a first option and I think this country would be much better off if McCain would have won the 2000 election. I think his choice of Palin as his running mate in 2008 was a disaster for this country.