The 64-bit host space is used by SEND to hold a cryptographic key as part of a method of securing neighbor discovery. 16 or 32 bits would be laughably inadequat

The 64-bit host space is used by SEND to hold a cryptographic key as part of a method of securing neighbor discovery. 16 or 32 bits would be laughably inadequate for that. It's also used as a passive defence against IP scanning, making it significantly harder for worms to spread by brute-force scanning. I admit it does make the address space look a bit lop-sided, but taking bits away from the host side (where they're being used) and giving them to the network side (where we should already have enough space anyway) wouldn't be a sensible trade.