"In a lot of ways, my personal belief is that Baen has now come to the conclusion that the free CDs are an experiment which has run its course."
Whether they have reached that conclusion yet or not, it is increasingly likely that the environment has changed enough that a different approach is needed. (The first edition/second edition model being one very nice example. As well as showing how the Baen folks keep finding new ways to play the ebook "game".)
The CDs (like the library) were primarily about promoting pbook sales with ebooks. But at a time that ebooks are 25%+ of the market, giving away entire backlists wholesale may be overkill. Other approaches, like rotating which volumes are available for free at any point in time might achieve the same results without cannibalizing sales too badly.
In the meantime, it may be wise to get the full set of promo CDs while the getting is good.