Oh, come on, the issue here is not the source control tool. Git is not magically going port your driver between versions. (BTW, I am a git fanatic, but that's another subject).
A successful compilation is not sufficient. Every release has to be exhaustively tested. Maintaining several slightly different versions of the same driver, plus tracking the yet different version which may or may not have been accepted in the mainline, is a lot of work. Especially if you have direct customers.
You absolutely need to maintain an out-of-tree driver. There is no two ways about it.
Look, this is not theoretical. This is my real experience and it has been that out of tree drivers are much more convenient for the users. If I need support for a piece of hardware, I download the module source, compile it and use it. No kernel upgrades, no backports, no crap. If I am going to install an out-of-tree driver, I actually prefer the mainline not to have one.
So, anything that makes it easier to maintain out of tree drivers would be good in my book. Of course everything must be GPL, and it would be nice if there was a centralized repository for these drivers so you don't have to hunt for them.