actually contrary to what certain other sites have claimed, there aren't that many secondary users - the only time you'll ever find winmx full of secondaries is when you aren't blocking, and those aren't users connected to you they are macrovision flooders
the average patched primary has 4 times the number of free slots that an unpatched primary, to give you some idea of just how many flooders are out there... (and most of those slots on unpatched primaries are from the small number of them using something like peer guardian to keep the worst of them away)
as long as TCP and UDP are set up correctly and you're running on primary you are making your system available to secondaries in the event that it is needed, although if it isn't needed then there's no great loss - better to have slots not being used than for there to not be any slots when they are needed...