From a journalistic point of view, used to work with sources, the mere fact that messages sent by the source are erased can be (very) counterproductive, as it often happens that the journalist, and/or the source, have to ask several questions, and thus exchange several messages, before being able to properly understand the big picture, or a tiny but very important detail.
As the messages sent are erased, by default, it’s impossible to look back at previous exchanges, except if they’ve been cut’n pasted elsewhere… which, in terms of security, is obviously insecure, as this elsewhere would probably be much less secured than SD is.
That said, would it be possible to :
. not erase, by default, the messages exchanged (but give the opportunity to the source to erase selected messages),
. or let the sources choose wether they want some messages to be archived for later use ?
. and, at least, warn the sources that messages sent will be erased and not archived ?