I have a steam phone (no web browser, never mind WiFi). Regardless, my phone network is O2, but I have my data package with 3 (the network built for data). 3's coverage isn't as good as O2 for voice, but O2 (at the time I was looking) didn't have a MiFi device.
If my phone breaks, I have only lost voice until I replace it (with another simple phone). If the MiFi breaks, I have only lost data. Either way I still have voice and data at home. Combine everything in a smart phone and then think: actually I don't need a land line anymore - iPhone breaks and you're up the swanee.