This may or may not be a bug or oversight, but it is something I've found to be a little overpowering in games:

Generally the game limits Generals to summoning a max of two minions for each category - grunt, siege, priest. However, I've noticed that this limit is applied as 2 of each minion type of each level - i.e. if I have 2 Monks summoned, sell the idol and upgrade to the Bishop idol, I get 2 Monks and 2 Bishops.

I've only stumbled upon the above combo thusfar, but I imagine it works with all Minion types and levels, feasibly allowing a general with 8 priest type minions. While lower level priests can't be replaced without buying new idols, I've found that the mass of healing makes them or me dying considerably less likely.