Remember that MS is making the Xbox act more and more like a DVR. DVR-like functionality requires being active 24/7. Which means even while playing a game, the console needs to perform this whole different task in the background.
I don't know where the 3GB reserved comes from but I can imagine if true it is exactly so that the console can perform these additional functionalities 24/7. In addition this also means no game can take advantage of all 8 CPU cores (maybe 2 cores will be reserved...?).
I knew this was what MS was going to do with the new Xbox, I was just hoping that they add dedicated hardware into the console that can take care of these new DVR like functionalities while the rest is 100% free for gaming. I guess in reality, whether you have separate 3GB for DVR functionality and 5GB for gaming, or 8GB total and reserve 3GB, the outcome is the same. The games will still only be able to use 5GB. And say 6 cores vs 8 cores. I'm just guessing on the number of reserved cores but there is no way MS can say a game can go ahead and max out all 8 cores.
EDIT: Of course let's not forget that the current Xbox 360 has 0.5GB memory. So if only 5GB is available for games then that is still a 10x increase in memory. But maybe 12GB would have been better if they need to split it up like that.