Also because the Archer/Bard is usually the last person standing in the case of a wipe. A level 3 Limit Break will have everyone back on their feet instantly.
And also because this game has enough DD's already.
While I agree their limit break should be support based I disagree there are enough DD's in the game. I have recently started to play Brd and I have to say, it is a lot of fun and I enjoy pushing tanks to their max. I feel entitled since I am a career tank and enjoy someone pushing me to my tanking limits. I made a dungeon run last night and me and the tank were having fun pushing each other.
Brd is a badass DD, and I really enjoy the support they offer when needed. The idea of producing some pretty decent DPS numbers and then being able to support the group for a "small timeframe" then go back to DPS seems really appealing to me.
WAR has access to enough enmity gear and enough hate-inducing JA to aggro baby jesus.
When I'm trying to decide what spell to cast I look at the mobs' weaknesses, check the day, check the weather and then calmly cast Thunder IV.