If, by chance, I have to be the first man in when clearing a house, I want that ability to put my weapon on burst and have the ability to lay down comparable amounts of lead to the AK47s I'll be going up against. Anywhere else, I'll be leaving my M4 on semi. I don't have enough ammo to be using up a full magazine in ten squeezes of the trigger.
Overall though, there is a reason that the SAW (M249) is considered the deadliest weapon in a 4 man team consisting of 2 riflemen (M4s), a grenadier (M4/M16 with M203) and a automatic rifleman (M249): volume of fire. As I was taught in basic, the SAW is responsible for the most enemy casualties in a fire team. Ask anyone who has been ambushed serving in the Middle East if their vehicle mounted M240s were used firing single shots or in bursts (can easily be shot both ways). Normally, full auto isn't needed, but when it is, by god I want it.