The question isn't whether or not full auto is necessary in belt fed guns as these are an essential part of military tactics. The question being asked is whether or not it is necessary in individual weapons like the M4 where full auto is rarely used in combat.
I'm going to say yes as they developed and continuously modified the SAW for use as an individual weapon. As for not including belt fed, that's not exactly a determining factor as it simply allows for more rounds to be fired without reloading. The SAW can take magazines, it just jams horribly. The M4 as it started (I believe) was only burst capable, but they came out with the A1 varient after for the Special Forces community so they could have full-auto capabilities. If the Green Berets and others want full auto, there has to be some situations where it is called for.
You don't really have to worry about us normal folks wasting ammo, as we don't carry that much to begin with. The last thing I want to do in a firefight is have an 'oh ****' moment when I have to scrounge for ammo since I blew it all away. A full combat load of ammo goes quick enough in semiautomatic as it is.
Please correct me if I am wrong on my technical data, but I do believe I am correct.