I do agree with the "WTF" notion of Shatner doing some really tough and complex stunts and then having a double for the BS falls. For example, in "Tomorrow is yesterday," Kirk's big fight with the Air Force guys was all Shatner. No doubling done at all. Yet, in "Operation:Annihilate" the bridge fight has a stunt double for Shatner and all he does is help push Spock down...and other actors are there without doubling. Unless Shatner was off shooting somewhere else, it makes little sense.
The Decker double in Doomsday Machine is actually quite good. A decent match for Windom. The real bad doubling is that for every scene prior to Decker being relieved, some smallish, wimpy looking guy was at the turbolift. When Spock calls him over, he becomes this big, tough, handsome dude.
Space Seed is funny as hell. The Kahn double is a total midget!
Kirk's double in This Side of paradise is pretty awful.