Too many customer devices don't support /31 subnets unfortunately, for example with Draytek we've seen an issue where it would accept the 255.255.255.254 subnet but we'd see a whole raft of connection issues making the connection unusable.
If we provide a dedicated IP for a connection where we provide the LAN we just put a single /32 on the loopback and NAT onto this which is obviously much more economic with addresses.