It puts you into a sort of CP debt, which lets you take full advantage of Good conditions that you otherwise would have gotten less CP back from. If, for example, your first move is Inner quiet, and the second turn the condition was Good, at best Tricks of the Trade could gain your 18 CP back. If you started with CZ, you would be down 66, and could hit Tricks for the full 20 (getting 28 back due to the CZ), and then hit Inner Quiet for a net cost of 10 CP that turn.

If your normal routine is to Inner quiet, then use Careful Synthesis 2 until you are 1 CS2 from finishing, then adding Quality, you could find yourself with several Good conditions that you could not otherwise take any real advantage of, due to a lack of CP spent. CZ, with its high initial cost, allows you to take better advantage of those.

Also, about Rumination : it becomes useless to you once you get Byregot's Blessing. BB is, once you have more than 3 Touches with IQ up and running, a cheaper, better, single-shot Super Touch. Once you get BB from Carpenter, you will never want to use Rumination again.