I agree with those of you who worry about 18, 19, and 20 year olds hitting the roads after drinking, but I do not agree with your conclusions. If the drinking age is lowered, those young people could go to bars, and student unions (at least here in Wisconsin) to drink. There would be some form of supervision for them (especially if there were laws like those Michigan enacted to make the bar owners and servers legally responsible for their service--for instance, it is illegal to serve anyone who is visibly inebriated).
Two of the most dangerous venues for drinking come as a direct response to the 21 year drinking age: the field party and the house party.
In the field party, young people drive to the event and drink as quickly as they can so they can get drunk but limit the amount of time they have a chance to get caught. At these parties, police scanners are used to warn the partiers if they have been discovered, and when they discover that their location is known, they pile into their cars and truck and drive away to avoid arrest. The 21 year old limit does nothing to stop the drinking at these parties, but it does send kids out on the road after they have been drinking.
The house party might be even more dangerous, for the students gather in homes for keggers, and they drink as fast as they can to maximize their "donation." These parties are so dangerous because they are held in the basements of homes so the police will not see what is going on when they drive by on patrol. This means that there will be a large number of kids packed into the basement of an old house--many times a house with questionable electric work In It--and there will be just one exit. If a fire would break out, I fear that the death toll could be tragically high.
I do worry about binge drinking, but I do not see how the laws we have now do anything to slow those who are between 18 and 21. There are, and I know you many of you will laugh, programs that have had good results on binge drinking. Schools like Northern Illinois University have developed education programs that effectively get students to resist the pressure to binge drink. Trust me, university presidents tend to not be "liberal;" they do want to fix problems on their campus, and student drinking is a serious problem. So this suggestion has far more to do with realism that some sort of liberal mindset.