Half of the TB kids on our team have captains and the other don't. The kids say they can get out of line easy or have stuff going on that the coach does not know about. One said a captain could help them follow rules and expectations, instead of their sneaking around coach and him yelling and getting in their business all the time with wrong facts. (In this case, it was/is daddy ball.) IMO, if you want to reduce the snide behind the back stuff, captains can be a communication channel.
What if the captain is part of the problem or lacks the the trust and respect of the other players? Or worse yet only has the respect and trust of half of the players? Now the team is divided. Do you then fire the ineffective leader and install someone new in the hope they will be acceptable to all the players? Or do you devote time and effort to fixing it? What do you do if your most capable leader is a Sophomore or Freshman? How will the Seniors cope with subservience to an underclassman? Probably about as well as an underclassman who has no respect for a Senior. Then again if the coach is not effective at selecting captains and is one where she would yell and get in their business all the time, maybe the coach should be fired? From my experience more often than not the use of "captains" is a catalyst for chaos.
Seems much more effective to allow leaders to emerge over time and let a natural order occur rather than selecting a figure head based on mostly on seniority. That way if someone steps up into the role who it turns out is not well suited you can refocus their energies elsewhere while maintaining team morale.