Does it make sense to only log 64 bit of IPv6 addresses for abuse purposes?
Logging the /64 will get you to the netblock owner, but if/when you actually engage with the real person that is responsible for that network, they're going to want the full /128. With statically-assigned addresses or DHCPv6, they are going to have logs of which machines got which IP addresses, and without the full address, the information you provide loses much of its utility.