Well, I'd certainly find it useful, since I much prefer to use a standalone client rather than the web interface. In fact, I almost never compose posts originally using the web interface, if I have access to LogJam.
Since auto-crossposting is probably the most significant feature in making migration to Dreamwidth easier, it seems allowing clients to use it would be a benefit, and I don't see what harm it would do. (Of course, it would require careful testing, particularly of edit-and-resave.)