deattilio
I've been on the fence about getting one of these, but I don't know too much about the M103 to begin with. I remember seeing one of these at the Mechanized Museum aboard Camp Pendleton and it was very imposing, even alongside an M60. Each time I see one of these the first thing I think of is "The Elephant Man" and "Mask" (old Cher movie).
Heavy tanks were developed by the US, UK and USSR as the logical successors to the heavy tanks of the later stages of WW2. Once armament and missile development progressed beyond armor development (main gun and missiles could kill any tank), the heavy tank just became a bigger, slower target.
A downsized version of the M103 became the M48 medium tank.
Since the Marines don't have as big a budget as the Army, they can't afford to not procure tanks they spent money to develop. Therefore they had to take them. The Army took the downsized M48 but knew it was to be a stopgap until the purpose-built medium tank, the M60, was in production. The Army did field a couple of heavy tank battalions.