Warmaster Horus
Explorer
And I've addressed that. But clerics and Paladins are different since they get their powers from 1 god only. While clerics and Paladins acknowledge other deities, they only worship the one they serve. A normal person pays homage to whichever god is pertinent to their situation. For example, a fisherman may say a prayer to the goddess of the sea before going out to catch his fish. When he comes back to shore with his bounty, and sells it. He may thank the god of wealth. He may then go to the tavern and pray to the god of luck right before he gambles his profit away. This would be common practice to most people, and while all 3 of those gods share from his prayers, none of them are giving him any of their magic.
A cleric or paladin of Bane, only worships Bane. They don't say prayers to any other gods for any reason. As far as they are concern, the other gods might as well not exist, and if they have anything to do about it, they will kill their worships and destroy the temples of other gods and make sure that Bane,is the only god worshipped.
They give an Oath to one god. That's it, according to 5e. Clerics most likely provide sole worship to a single deity but there's nothing binding a paladin to such a commitment. Is it common? Sure, probably, like the Bane worshipers you mention. Is it clad in iron? No.