Ion thrusters are routinely used on spacecraft - sometimes only to maintain their orbit, but many spacecraft now also use it to raise their orbit. It means the rocket can deploy the spacecraft in a lower orbit, which means you can build a larger spacecraft (or launch more satellites per launch).

Attaching ion thrusters to a rocket stage doesn't do anything useful - you would only use it once the chemical propellant is exhausted, but then you want to discard the stage to save mass anyway.