If someone wanted to use tor, he could just as well make VPN over it and set IP address to that in Retroshare. But then why not just use ftp or anything? Someone could also use I2P for this purpose, as tor is for hiding what you are doing in internet, while I2P serves the purpose of anonymous p2p through mixnet. But it's not good idea, now I understand that retroshare has different purposes - it is to create a safe sharing environment between a group of people who can trust each other. There is a competiton - OneSwarm, it does exactly the same thing + it's bittorrent client: http://www.oneswarm.org/
On the other way retroshare tries to be a platform with API to build more applications over it, not sure about OneSwarm. Maybe Retroshare should be broken into several components that could serve as a middleware to make other projects use it and thus become more important, of course possibility to pack everything into one installator should remain for ease of use.
Then we have to ask a question - what does RetroShare have over I2P? You could probably create it over that using it's networking part while having the GUI. In I2P you can make direct connections to friends if you allow it, and you can create darknets too so no outsider would use your connectivity.
