In a team with multiple generals, it is better for the team to have each general choose a different rank of priest because each player can be healed by each different priest type once per 10 seconds.  This means if two generals both have monks, you can only be healed by them once every 10 seconds.  However, if one of the generals has bishops and the other has monks you will be healed twice per 10 seconds.  If a third has priests, you will be healed 3 times per 10 seconds.

It would make more sense if players were given incentive to upgrade their priests as much as possible, instead of making an expensive upgrade actually hurt your team.

This is particularly problematic when you have a Sedna on your team and all ranks of priests heal for over 800.