TL/DR: There's really no argument - just carry a PLB for any non-trivial trip!
This is a question I've discussed with the leaders of SAR teams. They are unanimous that responsible use of a PLB makes life much safer and easier for both you and the rescue team. They encourage all parties to carry them, and particularly solo walkers.
I would argue that for any non-trivial project it's irresponsible not to carry a PLB.
Let's imagine a scenario where a solo walker does and does not carry a PLB.
Scenario 1: you don't carry a PLB
You are planning a 6 day solo walk on a remote wilderness trail. You are experienced and responsible and leave notice of your route.
Then you get an immobilising injury on day 3, with no cellphone reception. Here's how it plays out:
You have to wait at least 4-5 days before SAR will consider you lost and start their search.
Now the rescue team have to cover 150 miles of trail to find you, plus
there's the possibility that you might have lost the trail or tried
to bail out down a side-trail. This kind of search involves dozens
of volunteers giving up days or weeks of their time, and if conditions are
poor they may be risking their lives.
If helicopters join the search you are also risking the lives of the crew (there was a recent incident where a rescue copter went down with all hands lost).
It may take days to find you. Grim for the rescue team, and grim for you - if you have a bad injury this might be harrowing or even life-threatening.
Scenario 2: you carry a PLB
After your injury you simply press the button. The SAR team knows exactly where you are, and as soon as conditions allow they come straight to you and get you out ASAP. Simple as that. While rescue would have taken at least 4 days without the PLB (and often much longer), in this scenario you might well be picked up within hours.
Can any sane person argue that Scenario 1 is better?
We've been looking at a fairly extreme scenario (though it's not at all uncommon).
But the same principles apply on shorter trips or with larger parties. Used responsibly there is simply no downside to carrying a PLB. On shorter trips the benefits may be less dramatic but they are still significant. I was involved in a search in Glen Coe - a small but complex area in Scotland. It took days to find the walker, by which time they had died of their injuries. My PLB lives in my pack and I carry it all the time, even on day walks. I can't see any reason not to - in an emergency it's unambiguously better for everyone involved.
No need for debate - just get one and carry it!
Responsible use of satellite devices
Like any tool a PLB can be abused. It's emphatically not a reason for undertaking irresponsible projects on the assumption that you'll be bailed out if things go bad.
There was recent incident on the Pacific Crest Trail when a thru-hiker went into the Washington Cascades late in the season with storms forecast and against the strongest advice from local experts. Reportedly, his PLB gave him a false sense of security. Sure enough he became trapped by heavy snow and had to be rescued in the face of potentially lethal avalanche risk. This kind of behaviour is the height of selfishness and irresponsibility.
But SAR leaders tell me that this kind of abuse is rare. The benefits when responsibly used hugely outweigh the actions of the occasional idiot. They all agree that on balance the advent of PLBs is a game-changing improvement to safety in the back-country.
What about Spot or inReach compared to a dedicated PLB device?
A satellite communicator can also work if (a) You are 100% sure that there is reception throughout your route, (b) You are careful to preserve the battery life, and (c) You are careful to keep your account in good order. Plus with the inReach you can communicate the details of your situation.
But this is a compromise - a PLB is a much more powerful and reliable device for calling in SAR, and once it's purchased and registered there is no ongoing account to worry about. For communicators, the rescue functions are a secondary add-on.
In a larger party, the ideal scenario is to carry both a PLB and an inReach, and for deep-wilderness travel (eg Alaska or Greenland) a satellite phone is worth considering.
But if you're only carrying one device, the PLB is the way to go when rescue is your priority.