If you read the link in the first post, you would know, because it's described there and has a link to the NPRM. The Navy wants the airspace because they occasionally perform live fire testing across the airspace, from one Restricted area to the other. Currently, if any civilian aircraft are in the airspace (or will be soon), they shut down the live fire until the aircraft are clear. Apparently, they have some new system they want to test that doesn't have the capability for immediate cessation of operation (and I'm pretty sure I DON'T want to know about any weapons system that can't be shut off by pushing a button - sounds like a really freaking bad idea to me, but what do I know) and because of that, they want to be able to schedule testing and ensure that no aircraft will be in the airspace during that time period.I have no idea (and no "need to know") about what they are testing out there and why they want that airspace.
See, you're fighting a straw man here, because it's not airplanes that they want to test...But the reason for that gap is still valid. Anything they are testing out there can pretty easily climb up to the Flight Levels and just go over everyone else. Anything out there that really needs to stay down low to the ground can either use the existing large blocks of R airspace, or fly over to the Nevada Test Range where they have even more R airspace.