Just another data point that may not be that useful... you all have been riding for a lot longer then I have.
But last Jan, I was riding off road and it was slimy / slippery goo. Got half way up the hill and had to abort (it was, after all, a KLR-250). So I am off the bike beside it, trying to wrestle the thing to point back down hill. It was of course an awkward position (aren't they always).
I was wearing (for the first time) an old set of boots my neighbor gave me... very old, but top of the line MX boots in their day.
Anyway, they pretty much half immobilize the ankle in terms of rotating right and left, and make it hard to shift, etc. I was dealing with it, I'm a "wear as much gear as you can stand" type of guy.
As I was trying to point the thing back down hill, I hear and feel a little "pop" in my left knee. It didn't hurt that bad, but it absolutely had that "this aint good" feeling. I didn't fall or anything, I was just in a really awkward position.
I ride for the rest of the day, and it hurts, but I protect it. About 5 hours later, the back end steps out and I dab that same leg to stand the bike back up (no big deal) and *blammo*, the fireworks go off. Owwwwwwwwwwww. Shoot me, please, somebody.
A year later, I am almost cleared to ride again (Dec). The first pop was the ACL going, the real pain was tearing the meniscus (which is pretty much inevitable if you keep going after you tear the ACL before you get surgury).
I can't blame the boots, but in researching things I found that the frequency of ACL tears went through the root when ski boot manufacturers went from boots that let the ankle rotate right to left, to rigid boots that don't. Like those MX boots don't.
The reasoning is obvious (but not proven). The boots won't let the ankle rotate, so loads that are usually non issues because the ankle can turn and get out of the way are not concentrated on the knee. Obviously the knee is stronger then the ankle, but the ACL (and PCL, and various other $10,000 ligaments in there) might not be, and a boot hooked with an ankle that can't rotate out of the way has a LOT more leverage on the knee then it would have on the ankle.
So I'll never ride again with a boot that won't let my ankle rotate. I want a LOT of leather and trauma plates in my boots to protect me from pointy things and mashing, but they need to turn freely...
(which, near as I can figure since then... is the difference between enduro boots and MX boots... but I am still learning)...
FWIW.