The technical objections to actually using Android 64 bit for a non-Nexus device still stand. If it doesn't have more than 3GB of RAM and/or isn't running 100% 64-bit compiled code, the hardware thunking layer to run 32-bit instructions in a 64-bit architecture is going to be far more efficient than any software emulation layer that can run 32-bit code in 64-bit instructions.
I get why the Nexus 9 is running 64-bit - it's a Nexus, and Google needs a referential architecture in 64-bit because 64-bit really does represent the near future of smartphone evolution, and reference devices need to start getting into developer's hands NOW, even if there is no other technical reason for using 64 bits in the Android build
>3.5GB became necessary in the personal computing world years ago, and you need 64 bits to support addressing that much memory. This transition was anything but smooth and painless, and many corporations are still only offering 32-bit versions of popular desktop OSes. I have a laptop on my desk at work that has 8GB of RAM and an Intel Core i7 architecture, but only 3.5GB RAM are addressable because some of our 32-bit software *STILL* doesn't work in 64-bit glory. So I have a very sad 4.5GB of RAM that just sits there being refreshed by the hardware - billions of sad little electronic pandas that will never do anything, ever.
Smartphones are VERY FAST approaching the stage where they will need to have 4, 8, or even 16GB of RAM on board to support the workloads we are throwing at them. At least some of them.
That doesn't mean that any manufacturer of non-Nexus consumer devices has any technical reason to implement a 64-bit build of the OS unless and until they need more than 3GB of RAM on board. Benchmark and real-world performance will be slightly negatively affected. You introduce a whole raft of issues like hardware driver support, software emulation layers for 32-bit software and performance and memory addressing issues with 32 bit-optimized software even if it is compiled to 64 bits.
There WILL come a day, probably not this year, but very likely next year, when the first batch of phones come out with 4+ GB of RAM. It's important in the meantime to have some reference devices out there to start playing with 64 bit (hence Nexus 9, and probably other Nexii in the near future).
But the only reason a third party manufacturer would come out with a fully 64-bit-enabled phone with 3GB or less would be for sheer bragging rights. Which isn't to say it's not going to happen - it probably will - but there's absolutely no reason to be clamoring for it until it is needed.