I agree with Berto - it is not a broken miracle if the developers put it in the game. Broken aspects are when people use a lag backstab (the take advantage of their internet connection to get a cheap backstab in for a quick kill), or that they can quit when about to be killed.
When someone invades, they must be better than the host if they want their body back. Being better means:
1. Knowing the banish miracle is out there and
2. Being able to avoid it, whether interrupting the caster during the cast or dodging out of the circle.
I was in 4-1 with 2 blues once when I got invaded. He killed one blue, I summoned another. He killed that blue and was in the backstab animation on the 3rd blue when I ran next to him and cast banish. He couldn't hit me or roll out in time, so he was gone. In a game that's "kill or be killed," I chose to kill, even if it's by banish.
How many times have you been trying to get pure white WT, get invaded and die? In a game where being killed by another player can have dire consequences on your game, it comes in handy.
I think the people who say it's broken is because not enough players use it, so they never see it. I think it's one of the most useful miracles.
It's only broken when used against a red soul sign BP. If you summon someone, you're intent is to duke it out and see who's better. If the summoned BP is better, banish is a cheap way to make sure you don't lose a soul level.