The non-merging of the Nouveau driver - the reverse-engineered driver for NVIDIA adapters - also has a simple explanation: the developers have not yet asked for this merge to happen. Nouveau is not considered to be at a point where it works yet, and, importantly, there are still user-space API issues which must be worked out. Breaking user-space code is severely frowned upon, so merging of code is nearly impossible if its user-space interfaces are still in flux.

James Bottomley put forward another reason why a driver may stay out of the mainline even though the author would like to see it merged:

For the record, my own view is that when a new driver does appear we have a limited time to get the author to make any necessary changes, so I try to get it reviewed and most of the major issues elucidated as soon as possible. However, since the only leverage I have is inclusion, I tend to hold it out of tree until the problems are sorted out.

In other words, their control over access to the mainline tree is the one club subsystem maintainers have at hand when they feel the need to push a developer to make changes to a driver. It may well be that simply merging drivers regardless of technical objections (something which a number of developers are pushing for) will reduce the incentive for developers to get their code into top shape - and it's not always clear that others will step in and do the work for them.

On the other hand, the idea that in-tree code tends to be less buggy than out-of-tree code is relatively uncontroversial. So, for many drivers at least, a "merge first and fix it up later" policy may well lead to the best results in the shortest period of time. One thing that is clear is that this discussion will not be going away anytime soon; chances are good that this year's kernel summit (happening in September) will end up revisiting the issue.