I personally use them because I like OVERKILL......I have the set-ups to push the large cut mechs so I want to damage as much tissue as possible on the initial cut. I shoot nothing smaller than a 1.5" cut, and usually now a 4blade at that, and I shoot thru deer like they were wet toilet paper with KE levels from 75-88ft lbs. For example....the buck I killed last week from a ground blind had a rib centered in and out with a 1.5" 4blade mech and I found the arrow 45 yards past where it hit the animal.

If I get into the vitals its OVER regardless of what the head does after it stops moving. If I ever would have an arrow not pass thru, Enough damage is already done to put the animal down quickly, and from experience more quickly than more modest cut heads of any style (1 1/8" on down)that I've taken deer with. My heads are ALREADY cutting more tissue than a smaller diameter head the second it hits......I don't have to rely on the hope of the head moving around to get some extra benefits.....mine give it to me EVERY SHOT.

Now my set-ups are the exception rather than the rule when it comes to mechs so I don't expect many people to have my answer to the question, but that's why I personally use them.
Also expecting a regular fixed blade head to stay in and keep cutting is the exception as well.......plus if you are in far enough for the extra cutting caused by a running deer to be of benefit, you have already killed that animal before it does any additional damage. In that respect I don't consider a head that folds back up only doing half its job......would you consider a fized blade head that passed thru only doing hal fits job because it was not in the deer to cut additional tissue?
I don't consider the heads I use as a risk AT ALL.....in fact I have 100% confidence in the ones I choose every shot other wise I wouldn't be using them.

I do think that too many people are using the WRONG mechanicals for their set-ups giving them poor penetration and performance, and that is a whole other ball o' worms. As far as mechanical heads have come, education on how, what, when , where and why still have a long way to go.