It's fine as long as it's the only PM you use- as a training tool. Mine is now surplus to requirements because I have a p2m on the other road bike and a powertap for track efforts so I've stopped using it.
I happen to have a Stages arm (and a choice of Red or Rival cranksets) for sale :)
Is Stages still frowned upon, even if I only want to hire one for a couple of months to pace myself on mountains?