I have sometimes been asked for a one coat quick job. For example on a rental property.
My experience is that one coat doesn’t work. The wall might look reasonably covered immediately afterwards, but the initial opacity of the paint will fade over a few weeks and the wall will soon be “grinning” and shadowy.
Moreover, when a painter is attempting to roller a wall with one coat, he is likely to lay on a very thick coat, and so create a heavy roller texture on the wall. Thereafter, this texture will continue to show, under consequent paint jobs and a nicely plastered, flat wall finish will have been destroyed.