But is there really a point of having ARM64 on an ultra low cost system? Its not like you are gonna be using the increased bandwidth or large memory amounts that 64bits brings to the table on a sub $150 SoC, hell I seriously doubt the board will have enough bandwidth on its I/O to even saturate a 32bit pipeline.
I think the main point is to have a low-cost development board that people can use to port their software to AArch64 and/or test it on that platform (as said by the AC I originally replied to). I'm also sure that even with bandwidth limitations, the octocore will prove its worth when running our compiler test suite.
Additionally, the AArch64 instruction set has been redesigned from scratch and a lot of historical baggage and special cases have been thrown out (e.g. no more arbitrary changing the PC with half of the available instructions, and no more two different instruction sets with dynamic switching between them), so it wouldn't surprise me if over time AArch64 processors could actually become more power-efficient than regular ARM cores. Right now the AArch64 cores still include the ability to run regular ARM code too, but I'm pretty sure that over time this functionality as well as the entire regular ARM line will be dropped in favour of AArch64.