the downside of a stable API is more non-free drivers. the API being unstable is a strength in that it forces driver developers to use a GPL compatible license in order to get their drivers into the kernel, or it motivates the FOSS community to develop alternative drivers that are GPL compatible.

and, i believe i read somewhere that this is not an inability to stabilize the API but a conscious decision to not promise a stable API expressly because it creates the effect i described.