The API changing regularly is a real hassle for people building on top of it. But, the people building on top of it are also some of the people changing it most. And the API isn't nice and beautiful as it stands, with lots of horrible work-arounds for downstream users.
I'd suggest letting people change the API freely. But avoid making bikeshed API changes (those which are minor improvements at best, and really just a different flavour of the same). And perhaps loop in heavy API users when removing functionality.
Thanks, Neil