We do because:

1) when everything else is out, POTS still seems to work (at our house anyway). For us, some way of getting a call out is critical given family medical problems. POTS is ultra reliable.

2) We also get our internet via DSL, which needs a POTS telephone line.

3) Local POTS provider has a low cost pay per call service, and our POTS bill averages 6-8 dollars a month.