Whilst I agree anyone should be able to just jump in, not always but sometimes it can be the most infuriating experience!
The person that comes in with clearly very low defensive abilities, normally a caster/archer. As we know with most BT Captains, they tend to switch targets quite often. Said player then runs around like a headless chicken trying to shake the Captain off usually resulting in them persistently resetting the Captain to it's start position or in the worst case, pulling every other Captain in the area! They won't stand still or remain within range to be healed resulting in you having to chase the Captain all over the place till it switches back to you.
This process can sometimes result in the Captain actually taking longer to kill.
I'm definitely not saying, don't jump in. I'm just saying spare a thought for others there as you could make others very angry, very quickly! And I'm fairly certain I'm not the only one who has experienced this. I just leave if this type of player comes in.