What is clustering, you may ask? I'm referring to the surface fights when several players from the two teams are in possession of the ball and noone can get control over it.
Why should it be limited? Sometimes a team may even decide to keep the ball in a cluster in case they are winning and there is a very little amount of time left. It is a tactical decision, and it might even serve them well, but in my opinion, the rules should not allow it.
My first argument is that it's dangerous. One's finger or palm might be stuck inside while someone else frees the ball with a sudden, strong movement, so it carries a larger risk of injury compared to normal game. Also, UWR should be fast with a lot of passes. Clustering is absolutely against the true and beautiful nature of this sport.
Of course, you can't prevent surface fights completely. But if it goes on for too long ( 5 seconds, maybe ) the surface referee should decide referee ball to get the game going again.