I believe you should ban 'because I say so' as an answer. I dont know if it is good or harmful, but I generally say if I dont know an answer and if I know it is a question the answer nobody in the world knows, I will say this as well.
This. I remember my parents using this excuse all the time when I was a kid and it felt like such a cop-out. There is an answer for everything, even if it is "I don't know" or "nobody knows". I despise "because I said so" now and my wife and I don't use that phrase with our 5 year old, we always try to find the best answer to his endless list of questions.
The only thing I worry about there (i worry a lot...), maybe this dispise in the child can trigger to find out what really goes on. (Cannot explain well, its like you need to ban swearwords otherwise they wouldnt have any emotional power)