This might be common sense for some but there are so many uses for Mind Control on the Timeless Isle (make people drop off cliffs or aggro mobs etc). The most epic though (IMO) is mind-controlling people during the pirate ship event. I'm on a PVE server but usually when The Dread Ship Vesuvius shows up tons of flagged people come to kill it.
Here's what I did:
Stand under the boat so I wasn't getting targeted by fire and then MC'd a crap ton of flagged people to make them stand in fire. You can literally just go from one person to the next and keep them MC'd for a few seconds each while letting the fire do the work. I haven't tested whether you can farm blood coins this way but I'm guessing in order to do so you would have to MC and sw:death asap.
All Priests should try this even if it's just for laughs.