We know that lots of child porn exists. If you create additional scarcity by preventing the exchange of old/existing child porn (probably mostly 20 year old pictures, etc.) then you increase the chances of new children being exploited.
The legal adult industry has seen most of the profit evaporate b/c of the large amount of free content. The only people making money are those selling ads on top of existing content, which is often not even owned by the companies hosting it. The original owners lack the resources to enforce copyright law b/c they have no income stream to use to do so.
So rather than relying upon the idealistic notion that all exploitation and exploitative pornography can actually be successfully eliminated, it might make more sense to take a more pragmatic view about the forces of supply and demand involved, with the goal of reducing additional harm.
I can see no harm in anonymous data exchange (except copyright infringement) on an open society. But if you can order whatever you want online anonymously -- that could result in some deplorable stuff.
(That's why I think bitcoins will get shut down entirely by goverments at some point)
I do agree that the US Government will shut down bitcoin fairly soon, for the same reasons it doesn't allow people to create any other kind of currency.
I consider governments to essentially be criminal gangs that have achieved enough power to be able to buy legitimacy. Money laundering allows 3rd party gangs to start to claim legitimacy too, so it must be stopped. This is not to discount the good things governments (and gangs) accomplish, just to call attention to the good and bad done by both. Governments generally have elaborate propaganda and disinformation arms as well, and use money laundering laws to attack the funding channels used by competitors (described derogatorily as gangs).
How about an assassination market? Especially as far as present governments are concerned, this would be an enormous downside to the existence of an anonymous payment system.
Disclaimer: I am not declaring that assassination markets would necessarily be bad for humanity (I haven't thought about it enough), but on the surface it's definitely something that most people would consider to be a potential "bad" outcome of any anonymous payment system.