I could see it working out for some people - I get around 5 Mbps at home over 4G, and if my typical home data usage per month were low enough that the corresponding mobile data plan cost less than wired home internet, it could very well be cheaper. I imagine this would be true for many people who use the web lightly, and don't stream much video.
Comcast cable internet here is >$60/mo, and equivalent DSL is near that (although slower plans are much less), and T-Mobile's data plans range from $10 for 2GB (what I have) to $60 for 13GB of LTE data (after your data cap the speed is throttled, but you still get data). It wouldn't work for me, but for someone who used the internet mostly for surfing, facebook, etc., but not much video; it could pay off.