I don't know if, as a player, you would "need" rhythm to be effective, but when a player at any position is in rhythm, in "the zone", they are a more effective player than if not, at least in my experience and observation. Both CJ and JT have spoken about the rhythm factor here in 11W articles and I have seen other QBs talk about it at other times as well.
I know when I have coached defense, the goal is to break up the rhythm of the opponent, to keep them off balance, and when my teams have been able to do that our defense has been effective.
The rhythm of a player in sports is more art than science,I suppose. I would still say that if the players themselves are bringing it up then it is a significant factor to them.