[michaelt]

If I tell a guy to make me a table, and he puts his heart and soul into making me a beautiful wooden dining table, then when he shows it to me I tell him I actually wanted a glass coffee table and would he please start over from scratch, throwing his work away will be very bad for his morale.

That's not to say requirements can't change - of course they can. But they shouldn't change just because I forgot to make myself clear the first time.