I don't know - but it is privacy issues that have always basically prevented me from using sites like that too much...Now if you can somehow obscure not just real life identity, but IP address as well (because IP can for sure trace back to an ISP - and from there you get someone.. And even so all "anonymous proxies servers" are only private after the proxie server - again go through the proxie servers logs, and you got a true IP address anyway. So maybe I might participate in such a thing, IF I personally hosted, so if my machine was not online, neither was the server.. Reason - then I literally control all the information, and could delete anything I want, and wipe it clean enough that if it were a military drive, they would be comfortably enough that there if nothing useful there - to just give that hard drive to Alquida. But if I don't do it myself, I don't know it's cleaned that good....
Now yes, the messages could still be copied before deleted, exc. My main thing though would not be to eliminate any message - it would be to make sure it's not possible to ever figure out the real life person that put it there.