Ehh Neutral on this. I've AFK'd on the boss mob because too many people crowd around it. There's no point in fighting it when your teammates are cornering it and already slashing at it left right and centre. All I would have done is got in the way. Also, if I am very low on health and my teammates seem to be getting through the final waves without my help, I would generally step back as I wouldn't have wanted to die. It's not really selfish as I could have been the main fighter in earlier waves. I just need other players to cover for me. Just yesterday, I died two hits before the boss mob died. I was fuming but had I stepped back instead of fought him, I probably would have survived and received a mob head / nether star. If the minimum damage bar is set pretty low so that you can basically easily achieve this, then I don't really see the point in the damage bar at all. I understand the problems that other players have with this so I kinda support the suggestion but I do have my reasons against it. Either way, I wouldn't mind if it was implemented tbh.
No support, there shouldn't be a limit for how much damage you should do to a mob. I would say support if there is a way to kick someone for not moving for like 2-3 minutes, but players can just change their position around so they still do not get kicked.