For mistweavers, mastery increases the chance for your heals to spawn healing orbs, which heal players when run over or explode to heal the nearest player within 12 yards if they don’t get used within 30 seconds of spawning. For DPS purposes, this does absolutely nothing, so your mastery weight is 0.
(sorry, this is the post I had saved up for a really busy day, which came up sooner than anticipated)
Update: This is not true! See here for details, but mastery is currently negative DPS for crane DPS, due to orb spawning resetting the swing timer.