avik, I don't think you understand how complex these interfaces are.
Its not something you can assess in abstract form, i.e. until you've written a complete graphics driver, both kernel and userspace components you rarely know if the API you chose is going to be correct and performant. However you generally pick the 80% interface, go with that, and hope you can easily make the 100% interface on top of it later.
However unless a single group is developing the kernel and userspace drivers, generally that API is going to be useless and really constrain any one else. So we end up with a driver in the kernel providing an API that only a closed source userspace can exercise and an API that only a open source userspace can exercise, why should we introduce the first API at all.
I don't mind introducing reduced functionality kernel drivers that don't expose major userspace APIs to just serve as an example of how these GPUs work, I don't want a driver with an API commitment and no way for anyone to make sure the API continue working.