How does that factor into Shatner doing an entire fight himself in some episodes, and being stunt double heavy for simple gags in others?
As I said, there could be any number of factors involved, not just one. So each situation would be different and there's no simple, overarching answer. To know the specific decisions that went into each individual episode, you'd need to go back in time and see for yourself, or at least track down the motherlode of all production memos, or interview every last director and producer and stunt performer about each individual scene.
Anyway, I'm not sure a layperson is qualified to judge which stunts are "simple" enough for the actor and which require doubling. Some things that look simple can actually be genuinely dangerous without proper training, like falls. Or I imagine that the director and producers would prefer doubling any stunt that carries the risk of damage to the face -- you don't want to lose valuable production time while your star heals from a black eye or a split lip.
I was actually asking Maurice how an insurance thing would prevent Shatner from doing a light gag when he did more difficult stuff before and after.
As for what is simple or not, just check out the bridge fight in Operation Annihilate. All he really would have been required to do in the long shots would be to help restrain Spock with a few other guys. Pushing him down, really. This is the one bit that actually causes me to wonder, because it's ridiculously simple. This is why I am lead to believe he was not on the set that day, or shooting another scene, and came in for the close ups.