I always thought that since the stone rings are druid rings that they were Druid ports and that only druids could use them. Of course anyone can go along for the ride but it's the druids who have the skills.
Kinda the same way only wizards can use the wizzy portals...
Of course I could be wrong... In any case, druids are the only priests I play so I've never even tried to collect ports on a shaman or cleric. But the more I think on it the more sure I am... I regularly group with a mystic and she always waits for me to port us around...