When IPv6 IP addresses uses 64 bit prefix length, then the interface overlaying this IP address will not be considered for connection creation due the ping-pong issue, neighbour cache exhaustion (denial of service).
Other technical reasons for not using 64bit prefix length can be found at the following location.
http://tools.ietf.org/id/draft-kohno-ipv6-prefixlen-p2p-01.html#Recommendations
