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.