From what I've seen, facilitating proprietary drivers seems like the motivation of most people lamenting the lack of a stable ABI. An example of this being the comment I responded to; "Linux purposely makes proprietary drivers crap. [...]"

Discounting proprietary drivers under the assumption that they wouldn't be written anyway, what does a stable ABI afford us? Out-of-tree FOSS drivers? In other words, drivers that aren't good enough to be accepted into the kernel?