I'm afraid that you are confusing the terminology a bit here. R2 means not being tied to code that must be built with gcc 2.95 and not being tied to the r5 API or ABI. The 64-bit version of Haiku shares almost all of its code with the 32-bit version, so it is essentially only free from having to actually be built with 2.95, it still has 2.95 compatible and BeOS r5 API compatible source code, and is still limited in changing code if it would break the 32-bit code ABI compatibility.
As explained by @Barrett, as well as not breaking the API or causing changes in the 32-bit ABI, this means the 64-bit version can't use any new features of C++ from newer compilers, and thus the only real benefits are better code generation from new compilers and not having to carry around two sets of compilers and libs. Plus, I guess, basic 64-bit benefits like larger process address space.
I should add that in my first post, all I really meant is that I am looking forward to a time when we are not constrained by r5, I want beta 1 or r1 to be released and see what comes next! I didn't mean to start this debate!