I'm not sure who "we" is in your question, but that's what ARM Ltd. responds:
"In the desktop environment, a larger virtual address space is important for modern desktop software APIs that may come to rely on having a vast virtual address space for techniques such as memory mapped file I/O and sparse addressing (e.g. for persistent objects). AArch64 also enables efficient 64-bit immediate generation meaning less need for literal pools. A large program counter relative addressing range (+/-4GB) for efficient data addressing is helpful within shared libraries and position-independent executable." http://www.arm.com/products/processors/cortex-a50/?tab=Why+64-bit