Unless you change hardware often and have to call microsoft every two weeks because your legal version will no longer pass validation. **** that.I always found it easier to go legit with windows. It's a matter of $200-$250 every few years so I don't have to worry about the constant headache of trying to keep a non-legit version updated.
P.S.: 'Someone I know' who has a pirated version of XP has been using the same version for three years and it still passes WGA checks.