Yes or no. I've always thought it served the purpose of stabilizing the back end of the saddle in the turns. It seems to be out of vogue in the lightweight saddle. I need thoughts and opinions.

I always ride with one and I always ride with a breast collar...

The back cinch - tightened correctly - keeps the back of the saddle down & not "bouncing up & down" on the horse's back.  

Back cinch is dependent on rigging style and placement to ensure an even pull on the saddle tree.

Always yes! 

Yes always and snug fitting 

For running barrels, I prefer to use a back cinch. I like my saddles to fit as well as they possibly can, but I also want to minimize any "back slapping" that could happen during a run.

I also prefer to use a breastcollar as well, more for safety. If something were to fail on my saddle during a run (like the cinch), I like to think the breastcollar would kind of help save me.....

I use one but I don't like it tightened up real tight.  I want my horses back to be able to shape up when horse is getting on his rear end.

With out the back cinch, your saddle will become a catapult, launching you over the horse like a lawn dart. If tight enough going down hill help it from sliding forward. Also keeps pressure off the withers.

Used to use one all the time but have t in the past 20 years. If you are sitting down deep in your saddle in the turns your saddle won't be popping up in the back. If you lean forward in the turns instead if standing up in your stirrups it can't pop up.