"You want me to allot a subnet with 18 million trillion addresses to a link that will only ever use two of them? Are you kidding me?" We know all the arguments for what we get in exchange for squander: Easier address management with one-size-fits-all subnets; simpler address interpretation; scaling; flexibility.
But still. Only using two addresses out of 18 million trillion? (Saying "million trillion" is a lot of fun if you imitate Carl Sagan’s voice.) Well, ask yourself when a /64 is acceptable.
Most people would say they can accept it on a regular LAN or VLAN segment. All righty then. To be fair, let’s take a really big LAN. Say, 5000 devices. Is a /64 acceptable there? Yes, you say? So we’re wasting (1.8 x 1019) – 5000 addresses instead of (1.8 x 1019) – 2 addresses. The difference between 5000 and 2 relative to 18 million trillion is miniscule. It diminishes to practically nothing. If it were any smaller it would be the amount I’m being paid to write this.
And yet a /64 on a LAN is acceptable and a /64 on a point-to-point link is not. IPv4 thinking can twist our reason. All of this does not mean there are not reasons to use a prefix other than /64 on point-to-point links – it only means address waste is not one of them. In fact, there are dueling RFCs on the topic.
RFC 3627 makes its case right in the title: "Use of /127 Prefix Length Between Routers Considered Harmful." The central argument in the document, however, is not as striking as the title suggests. Here it is: