Mine is Xfinity Mobile, it requires you to be an Xfinity customer mind you, and if I wasn't it'd cost an extra $10 per line IIRC. You might argue that I get a cheap service because I'm paying for an expensive one, but I'd have the service anyways since at the time it was the only option for anything faster than 5Mbps. And if we use more than 1Gb, then it's another $12 no big deal, at the end of the day it was cheaper than what we were using which was MetroPCS (not exactly a major carrier)What MVNO are you talking about that's that cheap?
Here's an article showing other options, maybe not as cheap as $12 per gig for up to 5 phones, but there are still $10/month plans. Granted some of these certainly suck, and you probably are not going to get all the bells and whistles (and reliability) as a major carrier. But if it does what you want, why should you pay more? Not saying this is for everyone as your lifestyle may differ than mine (someone who's near wifi I can jump onto most the time, and when I'm out I don't need to do "high data" things like watching videos and what not)