I'm not sure whether people above are talking about the linux drives, or the binary firmware blob that runs on the MM chip itself, but remember, the drivers give you access to the firmware on the MM chip. There is a lot of firmware running on that chip which stays closed, which nobody really needs access to anyway - and that's the same for all MM chips (and network chips, and bluetooth chips and wireless chips and and and etc).
I would hazard a guess that 99.99% of people who would use this sort of board will be fine with the binary firmware blob. A lot of work has gone in to that code to make it perform extremely well, and there is no real need to have access to it - you won't be able to make it better (well, you might, but it would be extremely difficult as you would have to write lots of propriety assembler code - I've tried and it's not pleasant).
I'd have to check about the open source status of them, but I think the linux drivers will give you all the access most would need to the very high performance levels of the MM chip.
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