All cell phones can be located to varying degrees of accuracy via cell towers.
If the phone is visible to three towers, you can triangulate and get a good location.
If the phone is visible to two towers, you can narrow to two likely locations. If you have a history of different locations, you can quickly narrow down to one. You won't have that history if you turn on the phone just to make a 911 call and stay put.
If the phone is visible to just one tower - at least you've narrowed the search to the service area of that tower. Again, if you have a history of different locations indicating a probably path, that history can narrow the search area considerably.
In the case of the Kim family SAR in southern Oregon, single tower pings helped narrow the search area considerably.
Many cell phones include GPS, and most of those transmit GPS location through the cell system to more precisely locate 911 calls. As with all GPS devices, if the phone can't see the GPS satellites (e.g. deep forests, canyons, rock overhangs, inside buildings), the phone can't send a GPS position. And sometimes GPS locations can be off by a mile or more due to reflections and other problems. Still, transmitting GPS locations is much better than not having GPS most of the time (GPS use does reduce battery life).
Some people confuse cell tower locations with GPS. If GPS capabilities of the SpareOne Plus are important to you, I would ask them to directly confirm that the phone has a GPS receiver, and transmits GPS coordinates with 911 calls, versus simpler cell tower locations. Their web site certainly does not highlight that feature.
As to the value of DeLorme InReach SE versus SpareOne Plus - that depends on your own risk/reward tradeoffs.
If the SpareOne Plus keeps your wife as happy as a DeLorme InReach SE, and you don't care if you get found or not, then go for the cheaper option. People have been tramping in the wilderness for a couple hundred thousand years without cell phones or DeLorme inReach SE.
Just don't expect the SpareOne Plus to work in wilderness locations with no cell coverage. Having cell coverage at the top of a mountain does you no good if you are disabled at the bottom.