A reader directed my attention to an article in Massively about the new Star Trek Online Foundry, specifically the part where players used the Foundry to create missions that practice their jumping skills, needed for one of the game's raids. Sure, that would be one of the valid uses of such an editor for player-created content: Create training missions for developer-created content.
Unfortunately mission editors and MMORPGs based on player advancement don't go well together. Either the players are given very little freedom on how to create new content, or they tend to use any freedom given to create missions that give the maximum amount of reward for the minimum amount of effort. Paragon had to threaten players who used the mission Architect of City of Heroes / Villains with being banned for exploiting.
That is the curse of games in which character advancement and rewards have become more important than any notions of "fun". Add an editor and players maximize rewards instead of maximizing fun, because they can't even tell the difference any more. Given the possibility, they would create the big red button which advances any character to maximum level with best in slot gear in all item slots, in spite of that button being a thinly disguised "game over" function.