Yeah, this wouldn't be a terrible game mode to have just for fun but I don't think it would turn out to be very popular.
Look at the scoreboards on, say, battle.
You usually have a few very skilled players on both teams dominating. Maybe being free for all would change their playstyle, but I rather think they would do even bettersince they don't have to worry about team wounding.
This is important I feel because you would have a few people doing incredibly well, and they would likely ignore each other for the beginning of the battle while they killed all of the peasants. The game mode would just cease.
That's the problem, I feel that there should be more to promote teamwork in this game rather than individual skill.
Now.. a TOURNAMENT mode would be awesome. Set up teams and do like 10v10v10v10, eliminate the worst team, or players, etc.. Definitely would get behind that. You would get battle experience and possibly dueling experience in the same round!