Theoretically, this is awesome. If we had the ability to pay through a phone, it would give us an easy way to take care of the most awkward part of the process. If payment was regularly processed ahead of time, it might even make providers feel more comfortable with the visitor. I could even see this reducing the number of no-shows which might even lead to lower rates for all (I know this is wishful thinking, but it's not impossible).
BUT
Any method of transferring funds online must be attached to a bank account. That money has to be kept somewhere--whether it's a prepaid credit card, a huge financial institution, or some small, little-known bank. I'm pretty sure there are laws that require detailed tracking of bank transactions. For an infrequent hobbyist like myself, this wouldn't be a problem. I can have lots of small accounts all over the place and, unless LE targets me specifically, there's really nothing that points to me or attracts attention. A provider, however, will have a very distinct trail of transactions that could easily be identified and targeted as illegal activity. In addition, the IRS can access bank account info. Even if they aren't targeting someone for an illegal profession, they could target him/her for tax fraud or evasion. So, this is not really a good option.
HOWEVER
Today, Bitcoin is the currency of choice for illegal activity because it's essentially untraceable and easy to get. Cyber criminals deal explicitly in Bitcoin. If you ever get the cryptolocker virus, they will require payment in Bitcoin to unlock your computer. Personally, I wouldn't mind paying for services in Bitcoin. There's still the issue of actually buying your Bitcoins. You have to pay for them somehow, and that's probably going to be with a credit card. Still, once you have them, they are easy to transfer and difficult to trace--making them the best option for today.
This is all changing very fast. Next year there might be an even better option. I'm sure there are already other options that I don't know about