Not at all unusual for intake valve stems to be worn out on a MB diesel. The valve does not rotate (it is held in a fixed orientation by a pin sliding up/down in the guide to keep the intake shroud oriented.)

Wear is th econsideration, and how the engine will bedriven. Racing purposed argues for all new. Normal operation (anything less than WOT and redline) means reused determined by condition--worn stems, worn spring keeper grooves, warped, pitting, top too thin after re-surfacing--all mean use new one.