As per http://lists.w3.org/Archives/Public/public-pointer-events/2014JanMar/0104.html and some of the discussion from http://lists.w3.org/Archives/Public/public-pointer-events/2014JanMar/0122.html - I propose:

1) changing the first note "When two or more pointer device types are being used concurrently, multiple pointers are considered primary. ..." to "When two or more pointer device types are being used concurrently, multiple pointers (one for each pointer type) are considered primary. ..." (this was not minuted, but I seem to recall somebody suggesting this in the call)

2) adding a new note (after the first one, above), to clearly spell out the fact that mouse compat events are fired for every primary pointer - so if there are multiple primary pointers, they will all fire mouse compat events: "NOTE: In the case where there are multiple <a href="...">primary pointers</a>, 
these pointer will all produce compatibility mouse events.". In effect, this is implicit in the spec (as at the start of 5.1.2 we do say "Only a primary pointer will produce compatibility mouse events"), but this just makes it crystal clear, as a result of the previous note.

3) at the start of 5.1.2 we have "Authors who desire single-pointer interaction can achieve this by ignoring non-primary pointers." - this is perhaps incomplete in light of what we say about multiple pointers. I'd suggest adding at the end of this sentence "(though see the note in this section about multiple primary pointers)".  If we wanted to be even more explicit (but this would make it overly long perhaps), we should specify that authors should only consider the primary pointer OF A SPECIFIC pointerType...