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.
I disagree with this being the result of that. Generally, most campaigns do not manage their campaigns properly not because it's "hard" (if it's hard, then find a proper manager or don't do it at all), but because they don't care. Pushing all the spam into a singular sub-forum would not make our 'job' easier as the quantity would most likely not reduce as much. Keep in mind that they would still probably spam in other sections to farm up their accounts (higher ranks = more money).
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.
We don't have that much time to experiment. Action should be taken now.
Edit: Maybe there should be a 'no broken English allowed' rule. I know some might find it discriminatory and that's the downside.
That's near impossible to enforce consistently due to the nature of the rule. That one is based on subjective perception and would likely alter a lot dependent on the linguistic skills of every individual moderator in addition to their 'strictness' (some may find a post to be "too broken" while others may not). Generally, if you have really broken English then you should not be posting outside of your local section (you should be reading and trying to improve your skills).