It's not silly for people who are actually using this. I'd rather pay for the service than living with the uncertainty of getting a banner because you can't pay your CDN bills. That's really the only thing that prohibits me from using this service in anything useful anyway. It looks very nice so that clearly would be a waste.
I'll only do that, with the watermark, if it goes over what my bottom-rung Linode+CDN can deliver.