Hi sir,
Well first of all webmasters want to pay as less as possible :D
No serious, I think it's hard. Advantage of member based is that webmasters know what they have to pay you.
With paying per GB's it's harder for them to calculate the costs upfront. So that's why a lot of webmasters love paying based on logins per 24 hour.
So when you have hourly/daily stats of used BW, ppl would maybe switch to that, otherwise they would stick to per login.
Just my
Andre