You aren't playing protect right if you find it mostly useless outside of MEVOing when it makes a wonderful scout against Scarf Lando-T being able to either lock him into an EQ, and switch in a flying/levitate mon, or figuring out he is planning to use U-turn and nail the switch in hard. Same with scouting out a choice Scizor, locking it into BP and switching in Rotom-W. Lots of choice locked mons, usually surprise scarvers, love to revenge or check MDiancie is what I notice, and with SR protect makes a wonderful way to call their bluff (especially Lando-T so you can notice that lefties recoveries without giving him the chance to set rocks on your field.)
It has other uses as well in being able to lure and kill things that think they can kill Diancie before MEVO like Heatran/Bisharp/Keldeo - in some cases it can be clutch in wasting a turn of weather and of course baiting Mega Lop into HJK. Protect has a lot of utility if used right, especially if you play a volt-turn team because of the games it can cause and the constant switching which you can easily use to your advantage. Some players also like pairing it with Ferrothorn to wrack up passive damage.
They key thing I find when using Mega Diancie with protect is her weaknesses are very easily covered, a plain FWG core works, and since she baits a lot of attacks protect can ease prediction to the appropriate mon - many of which work well alongside her namely Rotom-W and Talonflame.