So while the phone is useful in like 99% of many LD rides,
That was good for a laugh. Thanks.
Let's just say that varies widely, depending on your location and your rides. If you're just doing cert rides and sticking to major interstates, your statement is true. If you're doing rallies, it's not the case. If you're off the interstates and especially out west between the Sierras and the Rockies, it's very often in areas of limited cell coverage, if any. Even on major interstates out West you run into areas of no cell coverage. The one time I would have called the RM during the '13 IBR, I had no cell coverage. I was on I-80, if memory serves, somewhere in WY.
If you're doing some of the less interstate named cert rides like the Great Lakes or Why to Why Not, Heaven to Hell, etc you will often be in areas of no cell coverage. And some of those rides require Spot trips to confirm your route since there are no convenient receipt locations. (Badwater, Mt Evans, etc.
Just consider what type of riding you are doing or want to do. If you're just doing basic cert rides and enjoying that, then the Spotwalla via cell phone might work just fine for you.