Marc,
We did stop at Robinson Crusoe Island on the Mariner Ring of Fire cruise. The reason passengers didn't go ashore on Pitcairn is that it would be impossible for the tenders to navigate the inlet to the island and with a full ship it would take too long to get everyone ashore with the locals' long boats. We also stopped at Rapa Nui (Easter Island) but bypassed Raratonga because the swells were too rough.
I have visited Papua New Guinea, Solomons and Yamamoto's headquarters on Rabaul and those are all fascinating places to visit.