Whilst I do agree with keeping things private, one argument I have in my mind, is...
If you're making a software, and were to hunt for a competitors "to-do" list, wouldn't you just look at your competitors forum and see all the people making "Suggestion" topics and noticing the Admin saying "This has been added on the todo list!"? I mean, avoiding a list of upcoming features just means that a competitor has to do a little bit of forum research and then they'll get their answers.
Again, I DO think it helps and adds to our suprise anyway, and I've noticed that almost all releases, Ashley never even hints about the features that will be in it and blows us away when it does get released. :P