Power Failures and DSL
Two things. The DSL-only type of service that doesn't include POTS is called "dry loop" in house, but AT&T didn't like the sound of that so they gave it some sort of cute term that the consumer wouldn't balk at. What that product name is I have forgotten, but "dry loop" should work if you ask for that.
Land line switching offices have back-up power systems of various types, usually depending on the predominant types of customers that are served out of them. A switch that serves predominantly residential areas will probably just have battery back-up, but a switch in a more commercial/industrial zone may have an actual jet engine to serve as a back-up power source. I've seen both.
There are also national security concerns around the landline telephone network, and anti-terrorist safeguards are a matter of course. You think the "hot line" in the Oval Office would ever employ a cell phone? Think again. There simply is not and never will be an acceptable substitute for the land line. The modern global consumer has come to accept products with inferior research, development and testing that don't perform as they should (we appliance buffs know this all too well) and cell phones are the worst example. Customers had to put up with crackling and poor transmissions in the early days of the telephone over a century ago. There is no excuse for having to deal with those same types of transmission problems with 21st century cell phone service, and the wireless telcos are laughing all the way to the bank as they reap huge profits on systems that require virtually no network build-out other than the towers, and virtually no maintenance compared to the copper wire land line networks, and apparently they don't have to answer to anyone about the inferior service their wireless systems are providing. They could never get away with such poor service on their land line systems. Between the FCC and the state PUC's, they'd have their feet to the fire if land line service was even half as unreliable as cell service.