Faster is better, where it comes to broadband. But so is a Lexus, right? But there's a reason we have vehicles in all sizes, optimized for different applications, at different price points.
We do different things with vehicles, and for most of us, money is not unlimited. Were it not so, perhaps most people would drive a Lexus. If one assumes there is very little a single cannot do with 15 Mbps, then a family can well benefit from 50 Mbps, if it believes it will have three or four users online, all at the same time, all watching video at the same time.
Lots of households will find that overkill, at least for the moment. In some cases, users can buy 50 Mbps service from Comcast, for about $100 a month. That's a better deal than $145 a month. But the issue for many users will be how much those users really want to spend for service, when they are paying their own money.