That's my point about efficiency. You want to practice ones that will give you the most bang for buck. Even as a pure striker, you only have so much time to practice.
A side kick does not have significantly more power than a front kick. Yet it takes longer to learn and leaves you more vulnerable in execution. Why use a side kick when a front kick will do? IMO a spinning back kick is
This is probably why you rarely see side kicks in kb/MT. Even in Sanda/San shou, side kicks rarely produce knock outs. Yet it inhibits punching due to the side on stance needed to throw one.
Without a doubt, a large part of it is because very few people know how to throw it. You should be able to throw a sidekick from a forward stance, if you train Sanda at least.
I come from an MMA gym, which draws its kickboxing from Sanda. The Sanda coaches are all about the side kick and advocate you to use it with the same frequency as a teep. On the flipside, I know tons of people who share your view and don't believe it's worth using much at all. My own views fall somewhere in between.
While there are guys from the gym who throw the sidekick with great frequency and have found much success even at high levels (Marvin Perry, Al Loreaiux, Albert Pope, Mike Norman, Sonia Mejia), I don't believe that's the right approach for MMA. If you set up and throw the sidekick correctly, the risk of you getting taken down is pretty low. In fact, I'd say it's wayyy harder to catch a good sidekick than a roundkick or teep. But that said the consequences of someone capitalizing on a failed sidekick is far higher in MMA than Sanda or another form of kickboxing, hence why I try not to throw it too often.
That being said, I consider it to be a worthwhile technique to have in my arsenal. In my past two fights, I've made pretty good use of it. It didn't do much damage, but it definitely psyched my opponents out because they just weren't used to it. Any disruption to your opponent's concentration is a worthwhile one, and getting stopped in your tracks, knocked back, or whacked in the face by a kick that you've never seen before is going to give you one more thing to worry about. Plus, it's almost always a guaranteed hit since people are unfamiliar with it. scoring points never hurts. I usually throw one per round.
IMO, it's not really a big deal one way or the other. If you can do it right, it can be effective in MMA, but probably not a game changer. If you can't, which is most people, they it's probably not worth the effort that it takes to learn.