As I mentioned before, if you're going to remove them from pretty much the entire forum it defeats the purpose of them in the first place so you might as well just remove signatures from the entire forum. Allowing them only in the Marketplaces would make those sections unusable.
The logic of allowing sig campaigns only in a smaller part of the forum would make it easier for campaign managers to maintain the quality of posts of their recruits. At the same time it would be easier for mods to clean up threads if needed. Spamming will be lowered overall and post quality will theoretically increase in the other parts of the forum.
But yeah, I agree with your idea of finding the right balance. Maybe it's good to test disallowing sig campaigns in the Bitcoin Discussion section for a couple of months and see how it goes.
Edit: Maybe there should be a 'no broken English allowed' rule. I know some might find it discriminatory and that's the downside.