Not only is stability and being able to integrate the drivers better within the kernel a key part of having the source for the drivers. If some poor person gets stuck using a piece of once 'in' hardware that the manufacturer has long since abandoned supporting - the issues can still be fixed. Or will have been fixed before the hardware became obsolete.
It also allows people who can code to help stabilise the drivers. Which essentially means the user has a better impression of the hardware. So the manufacture can benefit significantly from the source for that driver being available.
The reasons are practical - not only for stability but for longevity of support well after the manufacturer has long stopped caring. Having a stable ABI did nothing for Windows XP when the driver ran riot and took down the entire system in an all too common video driver BSOD.