Imagine the situation when you're the last man standing and you put on IWAY... and your character shouts "I Will Avenge You!"... and you have seen that a million times. Does it anymore feel like awesome? No.
But what if your mighty warrior would shout "To the end of us all!" or "Now I am mad!"

There are plenty of shouts to customize and that would definately add some color to the fights out there... even for PvP. You guys could really taunt the opponent; "Bite my dingaling, you noobs!"
Wouldn't that be something worth asking from ANet? It wouldn't affect the game in any other way than make it a lot more fun (I think).