You've obviously never had any experience with a full-auto weapon before if you think full-auto means necessarily spraying 30rds all over the place. My M16 is set up for auto, not a 3rd burst. Many submachineguns, however, have a 3rd burst limiter, so each pull of the trigger only yields 3 shots, not 30. Even on those weapons without a burst setting (like my M16's) it is quite easy to limit yourself to two or three rounds per pull of the trigger. Even with weapons with a relatively high rate of fire such as a MAC type submachinegun, you can easily limit yourself to 2-3 rounds per pull of the trigger. Its called trigger control; those of us who own and use these kinds of weapons learn it rather quickly. If your experience of automatic weapons is limited to what you see in the movies and on television you probably have little or no concept of it.
And if you think machineguns are particularly ill-suited for defense, why do many law enforcement agencies use them? Why does the military use them? I'm sure the folks carrying machineguns for diplomatic and nuclear security would be stunned to find out how inneffective their weapons are.
Is it possible to load up a machinegun and dump all 30rds in one pull of the trigger? Absolutely. But is just as possible for you to quickly pull the trigger on your pistol until its empty. Neither is particularly effective, and both can be just as dangerous if done irresponsibly or recklessly.
So tell me, how much experience have you had shooting machineguns?
Do you know what the average machinegun costs in the US? The people that can afford machineguns here in the US are very careful about them - they are very expensive and there is a far more extensive background check than for any other type of weapon. Do you have any idea what is involved in acquiring a full auto weapon?