Full Auto is simply great fun, that is enough to want them, add to that the fact that you cant buy them, and that the people should have the weaponry to counter the government and you have three really good reasons to want to own full auto weapons.
What makes them fun to shoot? Try throwing a Grenade into a trench, jump in and spray half a mag of 5.56 down the trench, Call the next guy forward, order him to throw a grenade past the next turn and clean it out with a half mag burst, continue commanding the cleaning of the trench and bunker and you will realise that it is a real good time. Or lie on the range with a FN MAG (Kulspruta 58 in swedish terminology, since it was adopted in 1958), along five other machinegun crews, pouring lead and tracers downrange at appearing targets, keeping them down. The song of the Machinegun, thumping in counterpoint to your rushing pulse.
The beltfed guns are made mostly for suppresive fire, and really fun to shoot in that role, 5-10 round bursts at the range is really satisfying.
Assault rifles like the FN FNC, H&KG3 or M16 is for the most part quite pointless to shoot in full auto mode on the range, though i have done that as well, full auto is useful in a targetrich or highspeed situation at short distances (clearing trenches or rooms in buildings for example), but at distances beyond 10-15 meters they really do a better job in singlefire. Full auto weapons or not should be my decision though, not the governments.