So imagine you queued up for a dungeon. Typically can take a while, because healers are in such need. You run half the dungeon, the healer hits lvl 80 and bails out. They got what they wanted, but now the rest of the group has to stop and wait for another healer to join, which could take just as long if not longer to get another healer.
If a player exits a dungeon or trial without a vote abandon or clear or lockout, they should lose all XP and items gained.