Why do Virtues and Flaws have to balance?

My experience has been completely the opposite. In WoD Games, I've only ever seen the virtue and flaw system used to score a few more freebies to get that one thng that the character needs for min-maxing. Similarly with Legend of the Five Rings (at least first edition, and that one thing that they need was frequently a virtue). None the less the virtue and flaw systems were a optional rule primarily for min maxing. In Ars Magica they don't seem that way by seperating them from the rest of character generation they become heir own thing and I've at least gotten far more enjoyment from them.