For me I think the main thing that team health bars on-screen would do is change the way people play the game based on the reasons outlined by Rado and Elaura. Ever-present health bars have a tendency to lead to prioritising heals based on respective values, and tend to work against situational heals (e.g. do you heal the teammate at 20% health who is running away from the front line, or the guy at 50% who is about to be focussed by two enemies? If your eyes are locked on the health bars, you might not even know what is happening until it’s too late)
The counterpoint to this argument is that they could be toggled on and off based on player preference, and I think that’s fine. What bothers me is that in a PVP setting team health bars actually help non-support characters just as much as supports. If the team has an obvious on-screen alert that their healer is under attack this will work against the opposition as it will make isolating and killing a healer that much harder. It is still possible, of course, but the change would put some players at a disadvantage.
TL;DR: I say add them for PVE but let people switch them off, but consider carefully what it could mean in a PVP setting

