Hi Andy. I don't see the harm in having a few Campaigns running. If somebody has a kit they want to build, or a theme they want to explore, why not put it out there and see if there are a few others of like minds. If there are a few more and a time frame can be agreed on, fire it up !! If not, it falls flat on its face.
I also learn a lot from Campaign members having a number of similar builds going on simultaneously, and in a focussed thread so they are easy to find. There is also the group research aspect, check out the P-40 Workhorse threads and what Mark (md72) has put together as a resource database.
I suppose one negative aspect is that it's hard to say no, and there is a tendency to enlist in several Campaigns at the same time (I'm certainly guilty there), which can lead to rushing a build and probably not getting the most enjoyment or the best finish that you could otherwise.
At the end of the day it comes down to self-discipline, of which I unfortunately have very little.
There it is, my 2 cents.
Cheers, D