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.
Because a lot of things that should be thrown into a volcano are used as routers. And routers need their interfaces configured. And the upstream interface needs to be in a network. And so that network needs a broadcast address, the router needs its own address, there needs to be a next hop address, and since you can't allocate 3, you do 4.
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.