Doing it this way, I get an understanding of their rationale and if I still think my idea is good, I can debate the worthiness of it against my employees reasoning. I feel this approach fosters a 'best idea wins' rather than a 'manager opinion trumps all'.
I agree with Derek's implied point that 'manager opinion trumps all is bad', but think it's a discredit to his staff if he doesn't challenge their ideas if he thinks he has something more worthwhile.