As long as my child lives under my roof, that will be a valid reason.
Sure, as they get older we explain things to them, and as teens our relationship will definately change, but that doesn't change the fact that as a parent, there are times when a simple "Because I said so" is all that is needed. It is also good for the child. Often times in life, whether at work or even worse in the military, your child might get an order from a superior, with the only explanation being "because I said so". If your child has learned to obey, simply because he is supposed to, he will do better. But if your child refuses to obey, unless an explanation is given, he might have a harder time.
For example:
Sergeant - OK recruit give me 20 laps!!!
Kid - Why sergeant?
Sergeant - WHAT!!! Because I said so, now make it 40!!!
or
Boss - I need you to get this report done right now.
kid - Why?
Boss - because I said so! I am the boss, and you are the employee, get it?
I get what you're saying - believe me I do. But I also think that there is a danger in children being too compliant.
There was a story on 20/20 recently, about an 18 year old girl working at McDonald's who was stripped and sexually assaulted for three hours by her boss and her boss' boyfriend.
http://abcnews.go.com/Primetime/story?id=1297922&page=1
Bizarrely enough, the manager also claims she was just "obeying authority", a voice on the phone who claimed to be a police officer.
Neither the victim or the manager seemed to be able to stop and think "I don't
have to do this just because someone tells me to."
Even in the military, soldiers are not required to follow an unjust order.
I also think that "giving reasons" is valuable because it shows children that mommy really does know what she's talking about, she doesn't just make this stuff up.
There are some rules that, if they are broken, will only resulting in mommy being unhappy - for example, if my son forgets to make his bed. The reason for making the bed is because I said so, and if he doesn't do it, I will be unhappy and he won't get to play outside.
But there are other rules, like not running away from mommy in the parking lot, or not answering the door to a stranger, that could have far worse consquences than just mommy being mad. The reason for those rules is because I said so
and because they could be hit by a car or let a dangerous person into the house. If those rules are broken, somebody could get very badly hurt.
I think it's to my advantage to give that extra motivation so that when kids behave like kids and "forget" the rules, they will hopefully choose to "forget" one of the critical ones.
Obviously there are different responses to different children - my girls are 3 and 10, and I don't treat them the same way in hardly any matters of discipline.