Well, maybe this doesn't answer your question, but: Even one punch or one kick, is not just 'one action'. Hitting it requires a set up, for instance stepping in, feinting, and then an exit or defense. A single attack, if it's really just one attack, probably wouldn't work (twice) against an experienced opponent.
So in my opinion: To increase the chances of you hitting something, and getting away safe, 'combinations' are a superior way of looking at striking. If you include footwork, head movement and feints in your 'combinations', you can have combinations where you only throw one punch, but it'll catch your opponent harder or safer.
In short: Throwing one strike is easy, but predictable, and not very effective. Once in a blue moon you'll land a hard, lucky shot.
Combinations set you up for success, both offensively and defensively. An opponent has a lot more difficulty with (good) combinations than (good) single attacks.