I don't see pressing 911 on a Spot or InReach any less serious than triggering a PLB.
From the SPOT documentation:
ALERT 9-1-1: Use this function In the event of a life threatening or other critical emergency to notify emergency services of your exact location and that you need assistance.
HOW IT WORKS:Once activated, SPOT will acquire its exact coordinates from the GPS network, and send that location along with a distress message to a GEOS International Emergency Response Center every five minutes until cancelled. The Emergency Response Center notifies the appropriate emergency responders based on your location and personal information – which may include local police, highway patrol, the Coast Guard, our country’s embassy or consulate, or other emergency response or search and rescue teams – as well as notifying your emergency contact person(s) about the receipt of a distress signal.
A PLB 'how it works' example is at
http://www.acrartex.com/landing/rescue
For me, when I get to the point of pushing 911 on any of these devices, I'd push it on all of them. My take is I want the best possible chance of response. I think a PLB is that device, and I want it in my jacket. Tracking has it's applications too, and for that I could see having an InReach or Spot AND a PLB. If I had both, I'd likely push 911 on both if the circumstances were dire.