There's loads of stuff from open guard that can do you more harm than good. Half guard in BJJ is a good attacking position, but in MMA is maybe just as bad as being mounted. Deephalf guard, inverted guard (if you don't attack the legs fast). Closed guard is pretty neutral, guys on top don't get triangled all the time nowadays and fighters are more and more able to defend against strikes with closed guard and butterfly guard. But there's still a shitload of stuff that fighters didn't really explore in grappling from your back with strikes.
What I don't understand, it's why there's not more single X entries to attack the legs. If your stuck on your back with a guy standing over your butterflies, you can just enter into SLX and attack the legs from there. If you don't manage the sub, you'll have a scramble to get back up and it's low risks because it's tough to GnP someone when you're on your ass with a leg locked up.
Right now, the top leg grapplers doing MMA are generally the best grapplers and are pretty much always on top, and like Danaher said, he won't let go of a top position to try a leg attack in a MMA fight. I don't think we'll see Tonon pull guard on purpose to attack heel hooks.