I have a triple on my Trek. The advantage is that when and if you need it you have it. The only disadvantage I can see is the "look" factor.
If the LBS is a reputable dealer he will support you after you buy the bike from him. Really, there is not all that much difference between a road and mtb as far as service goes. If this guy has a killer deal jump on it. The Lemond is a great bike.