Why, oh why make it two units connected via wireless?!
You take an existing GPS product and just add a few lines of code to talk to the SPOT. No new case design, no tooling costs, no CE/FCC approval and you don't have to carry two different models.
while if SPOT made this as a single unit they would be competing with both the established GPS makers and their own unit.
The SPOT puck is then just an off the shelf globestar modem, a bluetooth chip and a battery - very quick and easy to bring to market. And now you can add SPOT to any GPS (or smart phone) maker's unit.
It's not a very difficult market to enter, the hardware is pretty simple then you just need to sell enough to pay for the operation center.
But there is a first mover advantage, with safety gear people naturally go for the brand name market leader which seems to be SPOT.