The SPOT concept is a good one, but it presumes you have attentive friends who can respond to your messages. Although it does connect to the GEOS private system and through them to SAR.

According to Bill (who did the test on Trailspace), you are right about the different systems-they are on the same satellites though, although voice requires more bandwidth and power than the digital signal for the SPOT.

Operating a private satellite system is no cheap deal and they don't last forever, which is why Globalstar is having some problems-they have to replace some of the satellites.

One caveat from the GEOS website-if you sign up for their insurance policy for private rescue services, read this first-
PLEASE NOTE This benefit does not apply if:-
a) your situation is caused by circumstances such as a forecast change of weather conditions, which could reasonably have been anticipated at the date you started your trip
b) you have NOT made adequate provision of resources or training or competence needed to complete your planned trip.

Hmm, wonder who gets to decide that?