As I stated, my usage does vary month to month so having a roll over plan would be a huge benefit to me and many others.
I have both Verizon and AT&T accounts (both needed as I live in an area with no broadband and sometimes Verizon and sometimes AT&T are not working) and I can tell you the AT&T account is much easier to manage data on a monthly basis.
With Verizon, I never get to use my full 40 GB as I have to unplug beforehand to avoid overcharges (and yes, I have the 75% and 90% alert set.) I end up losing 3-5 GB monthly (so I really have a 35 GB plan!)
Maybe the FTC or FCC should look into this? Hope some day, Verizon will see to treating their customers better with service options that help