Having a 21yr old drinking age encourages reckless drinking at house parties. At a bar, you will at least be cut off by the bartender or thrown out by the bouncers. Either way, it teaches to be a little more responsible when drinking.
Another problem is the second we turn 21, we forget the cause because it no longer affects us.
We're continuing to allow our kids to be arrested and getting criminal records for doing what young adults are supposed to do: have a good time.