Indeed the usage of rifles by well trained soldiers is almost entirely on semi (single round, semiautomatic) mode.
Allows for accuracy, prevents failure and saves ammo.
That said, combat is dynamic, and scenarios vary.
A patrol can very quickly turn into close quarters combat.
A force can find itself in need of short suppressive fire bursts and no access to a light machine gun, etc.
This versatility is the main reason for the prevalence of the assault rifle.
A saying in the IDF goes "Battle is the kingdom of uncertainty", accordingly, the modem combatant must be capable of accomplishing more with less.