Do forgive my necroing an old thread, but rather than making a new one on the same topic, I just wanted to express my excitement over seeing this feature on the Spine waffle's list of intended enhancements. I know it could still be a long way off, but it's something I find myself wishing for in a great number of my animations, so any notion of it on the horizon is great news.
I did independently come up with the separate X and Y bones workaround, which alleviated a lot of my stress over getting smooth curves for a few cases, but having both values belong to the same bone would be much more straightforward and easy to work with. It's also one of the first things I tried to do in Spine, in coming from a 3D background, so the split option will ideally make things much more user friendly for the majority of new users who give up on getting a smooth curve rather than finding this thread or coming up with the separate bones workaround.
I'm sure it's far from the most important feature on the to-do list (especially given the workaround), but I definitely think it will be well received when it comes to pass. In the meantime, thank you to the whole team for all the constant hard work and updates that have been making Spine noticeably better every day!