Agreed! You would have to be paying for more than you need all the time to get any rollover when you need it.
No matter what carrier you have, the answer is to have a data plan that is large enough to cover your use.
I Bought a Nexus 6p, so I looked into Fi. It's coverage is questionable, and it wouldnt save me a dime because I use enough to data to rack up large charges. Basically, the per GB charge on my shared plan is much lower.