I think Usage Based Billing would not be the big deal it is if the cost were actually in line with the actual price of transferring the bandwidth.
For example, I'll be generous and say 10 cents per gigabyte. So, so heavy usage at 200GB per month would come to $20. However, the price the big ISPs are charging is significantly higher.
Add to the fact that with richer media content, we're dealing with far bigger files. For example, I recently purchased a video game online that led to an 8GB download.
And it is detrimental to cloud adoption, see for example this story about the massive bill journalist Matthew Ingram ended up thanks to storing his photo library on Amazon's cloud: What Happens When the Cloud Meets a Bandwidth Cap