I will say my appreciation for leg locks and the evolution of my BJJ game including leg locks was from Sambo Steve and a Sambo leg lock seminar he had in NYC. Even though knee reaping is illegial in Gi BJJ, I use it for control along with a full leg lace in No-gi. I also found this useful in Gi, having this base opened up using more BJJ-legal leg entanglements like Marcelo Garcia's 1-leg X guard and Ryan Hall's 50/50 guard. The key really is the leg entanglement more than the actual submission. I like to transition to whatever leg entanglement I can do (usually depends of whether I can reap the knee) and once I have control I go for whatever submission is legel/open. For example I will transistion into a leg lace regradless for control, its really independent of the straight ankle lock, toe hold or heel hook whichever are available. Additionally, if for wheatever reason I cannot find the submission I can use the leg control to stand and pass guard.
Honestly, it doesn't bother me that other grapplers have this giant hole in their game, its better for me :)
Already many great answers here. My POV as far as my experience:
MMA matches: A failed leg lock is usually going to result in you losing superioir position, and potentially the round. Much safer to just punch you from half guard than to try and reverse-cowgirl you into a kneebar or toe hold.
I'd say you see a lack of it in a lot of grappling due to playing for points, and BJJ being the dominant grappling art at the moment.
if you're interested in leglocks/defense in mma check this out too: