As was already pointed out, it is probably better to use one dragon, but generally if you have arranged backup, you are trying to make life easier for the backup.
Often this involves swapping before risking a mage supershot, and sometimes swapping immediately. (A big player taking a base you cannot REQUIRES rage to succeed often, and they start with the rage you leave behind)
The reasons for swapping immediately are mostly to avoid mistakes. Also as you noted the healing time will be much lower with 0 damage taken. But mostly new players can make a lot of mistakes and a big player supports many players who may make mistakes. It can just be easier to set a broad rule. Really only useful for War runs and atlas PVP.
If you are just vicious you can also swap just before death, but all this does is deny the defender credit for taking down a dragon and some MVP stats and maybe some defense rewards. I believe self destruct also avoids counting as a killed dragon.
Lots of teams manage their members differently, and players (even at higher tiers) can be surprisingly tricky to manage. Akin to herding cats without a laser pointer.
I can’t imagine it would not hurt to ask your leadership why they have that rule. Good communication is the cornerstone to a great team. Half the battle
Is sometimes getting the players to reach out and be comfortable asking. (Both leadership and other players)