[tongue firmly in cheek]
If it's true confession time, I hate warlocks. There I said it. I think they're stupid, over-used for multiclassing and and scream "Ooh! Look at me being all edgy and dangerous with my dark pact!"
Give me a break. The moment there's a hint of sulfur in the air and the debt to the dark power coming due it all turns into "Why do you hate warlocks so much that you have to punish my PC?"
I'll tell you why, Bob. Because you aren't dark or edgy. You're an accountant. The closest you ever got to dark and edgy was a "Born to be wild" t-shirt with a penguin on it. You only want to play the guy with a dark secret and a debt that will come due some day because you know it's not really going to happen. The devil may go down to Georgia, but he's not visiting your version of Faerun any time soon. Cthullhu? Yeah, if he notices you siphoning power the world will be plunged into chaos and madness but you know that won't happen because I don't want my campaign to end.
So suck it up, Bob, and play a real PC.
[/tongue firmly in cheek]
Okay, in truth I just ban most patrons for thematic and personal preference reasons. I don't want evil PCs, I don't want a corrupting influence. In my personal experience, most warlocks have just been a couple level dip to get some kewl powers.