Teenagers are going to go out and do It anyhow. That is the bottom line. You can teach them right from wrong, tell them stories about drunk driving accidents and so on, but they are going to make their own decisions. I would rather my child be open about it and do it in front of my face rather than go out behind my back and get drunk with a bunch of friends. The last thing I would want would be for my child to be drunk behind the wheel and take someone else's life or get in trouble with the law. I would rather them be at home and drink where I can still have an eye on them.
Obviously if you're allowed you're child or children to constantly get drunk on weekdays or weekends or both, then that is a problem and unacceptable. However by allowing your children to drink, you are helping them mature. From personal experiences it's much better. My mum lets me drink in the home and she gives the very valid reason that "I would rather you drink at home than go out and get so drunk you walk in front of a car." And she's completely right. At home, children can be supervised. If you tell your child "you can't drink" then it's very likely that they are going to drink, and in excess sometimes. Obviously not all the time. I think for a parent to introduce their child to alcohol, it allows them to explain the dangers more clearly, and to help them mature away from silly situations and peer pressure.
There are age limits on drinking for a reason, it is very unhealthful or the undeveloped brain to be drinking alcohol. Just because they do it at home it does not reduce the danger. Allowing a child to drink in the home is abuse and neglect. This should be an arrest able offense.
I'm sorry if you think "it's better for them to drink here than outside", but providing alcohol for your child is enabling the consumption of alcohol. We have a drinking age for a reason: alcohol can damage brain development. If somebody drinks underage, whether or not they believe in the law, and know that it's illegal, they should be punished.
It is not okay for parents to allow under age drinking in their homes. It is better than some alternatives but better should not be a substitute for what is best. The underage drinking can still lead to a lot of unnessary issue so the real solution is if underage then no drinking.
In general, parents should not allow underage drinking in their homes because it sends the wrong message. If it involves just their own children, go for it. But there are too many horror stories of teens killed because of drunken parties hosted at the homes of underage kids who have access to alcohol. Irresponsible or unknowing parents shouldn't have underage drinking in their homes unless their children are 18 and older.