Yes. PLBs are a very important thing to have for backcountry/wilderness exploration and offshore boating.

[1] https://www.rei.com/learn/expert-advice/personal-locator-bea...

I spend a decent amount of time in the backcountry--and more likely than not I have no reception. In general, if I can't see buildings my phone probably doesn't work. (Note that "see" should be interpreted broadly--looking down from a mountain you can rarely resolve individual buildings, but you can see it's developed land.) Major roads will generally have service even if you can't see buildings, but secondary roads rarely do.

20 minutes from San Francisco there are dead spots. I go hiking all around the Bay Area, and while it’s gotten much better in the last 10 years, there are so many national, regional, and local parks with huge dead zones.

Most rural areas in the US that aren’t near a town center or major highways are pretty spotty.

Quite a bit actually. Even highways out of populated areas won’t have any network coverage. Also the same reason satellite radio is still popular and sold here.

With no research, I can give an emphatic "yes."