The obvious problem besides the added complexity for users is that I suspect there is relatively little "background" traffic usage. Almost all of it is people watching streaming video or interactive downloads. I doubt things like software updates that can be scheduled add up to much.
It would be interesting though. I wonder if it would lead to services promoting their download options more (example auto-download likely videos every morning).
Comcast charges $0.20/GB for coaxial. Verizon charges $10/GB for LTE. My actual cost for hosting side projects is below $0.50 per 1 TB. It is unreasonable if they are going to fuck with your traffic, tamper with your HTTP payloads and inject scripts into them, deprioritize certain data and still charge multiple orders of magnitude more per GB.