The trouble is that Shax might not have a reason to actually come back to the island once she's been given a way back to the mainland. Stay among the non-humans who are hiding and trying to stay quiet, or go to the mainland where there are swathes of enemies for her to fight for as long as she wants.

If it got to the point where Shax was needed to go to the mainland to get supplies, there would have to be some kind of reason for her to return to the island. Not to mention we have to get to that point first.