The title is a false dichotomy; you should be both, if you're only valued but not useful you're part of this echelon of seat fillers who get fired at layoff rounds to cut costs.

Usually the valued ones are in charge of hiring and firing.

From King Solomon's Mines, there's a line from the indigenous "bad witch" when she's trapped the protagonists in the jewel hoard cave - something like "What will white man to with the jewels now? Eat them, perhaps?"

Assuming you wish to be valued more than you are useful.

So, basically, be super rare? And hard to work with?

That's deep.

Diamonds that are actually useful are quite cheap, compared to the useless ones.