Just to be clear, Shiirn's suggestion is Submitter > highest guest diff count.
Currently and what I think should stay a guideline is Submitter >= highest guest diff count.
Thus if 8 people map 1 difficulty each, anyone can submit (they will automatically have the "most" influence on the map due to organization). If one person maps 2 difficulties, that person has to submit.
In a collab effort, if there is only one map, it's fine. This should stay as a guideline however, as some maps are initially made as a single collab difficulty (for approval of course). The uploader might not want to make a 2nd map. If a guest difficulty comes along, the uploader would have to turn it down, as they have already submitted it in their name, and the guest mapper would have contributed the most to the map. I would also support the fact that it is for approval that it's easy to ignore this guideline. I'm pretty sure this is one of the problems Natteke is worried about. Since it's for approval and score doesn't count, however, it would be better to combine the 2 maps into their own single beatmap rather than get 2 separate beatmaps, 2 separate downloads, and 2 separate places to store the map. Of course the same could be achieved by dropping the guest difficulty as an unranked diff.
I'm fine for breaking this guideline under approval under those circumstances, but for 99% of the time it should be applied.