While it might not be a powergamer's favorite choice, I like to give my (human) monks a level of cleric, just so they can use priests' (heal critical, restoration, remove poison...) scrolls.
A monk's speed (+dash) makes her very maneuverable on the battlefield, therefore quite a good " combat medic"!
I know that didnt answer your initial question (have yet to playe a cleric of bane), but Ill say: go for it.
You dont loose much power in the long run by giving your monk one level of cleric, anyway.
Other points to consider (besides the scrolls):
- Items. Well, I dont remember if there are any "clerics only"-items in the game, which would be of any use for a monk...
- Quests. As you already said, Banites get an extra quest (Ilmaters too; dont kow about others), which means extra exp and other rewards.
- I some situations, even a level 1 cleric spell or ability (albeit a very short one) that you party's cleric wouldnt have to cast on the monk might be useful; say "protection from evil".
That's all; cant think of any other reasons for/against such a build at the moment.