I did a liitle googling and found a website entitled "Equipped To Survive" with a lot of FAQs and answers. Here's one that seems relevent.

"Q: When should I use my PLB?
A: Every PLB has a sticker that contains the following warning (or something similar): “ONLY USE IN SITUATIONS OF GRAVE AND IMMINENT DANGER” Authorities understand that the seriousness of an emergency can only be determined by the individual at risk and they are not likely to second-guess someone who sincerely believed they were at risk. In general this standard would mean that there is a situation that could result in loss of life, limb, eyesight, etc.

A PLB should be the last line of alert. If available and there is time, attempt to use other means to summon assistance first, such as a cell phone, satellite phone, or radio transceiver.

In general, experience suggests that many who did not survive an emergency waited too long to summon aid. Do not hesitate to summon aid immediately when a situation becomes so serious that a successful outcome is unlikely without an incredible amount of luck."

So, for example, if I hurt my back so seriously that walking out might cripple or parlyze me I'd use the PLB.