I don't know how much of an excuse it is. The reason people who get drunk talk like slurring morons and stumble around is because their CNS is highly depressed. It's rather scientific. Having 1-2 drinks doesn't put you in that condition (it gives you the relax simulated state that a mild does of a CNS depressant does), but people who drink aren't champions at limiting themselves.
Now voluntarily putting yourself in that condition is no excuse for the awful things you may do while in it, but the condition itself is very real.