Because I Said So!

This morning on the Early Show they had a segment about common things that parents say that can be emotionally damaging to children. The #1 saying was: "Because I Said So."
We say it, our parents said it and probably their parents said it. They said that when we say it our children hear something like "It doesn't matter what you think."
What's wrong with that? I completely disagree that it is not something you should say to your child who constantly challenges you and asks why at every turn. How is that different from their teacher asking them to turn in an assignment? Or their future boss asking them to work on a project? Not everything requires an explanation. Obviously you should not say it all the time, but sometimes children do not need to know the reason, they just need to obey.