It made more sense when you could assume each machine was tended by a sysadmin who was responsible for running it, so having a quick way to reach that person in

It made more sense when you could assume each machine was tended by a sysadmin who was responsible for running it, so having a quick way to reach that person in the Net's distributed "phone book" was a fairly obvious thing to do. Spam... existed, actually, but it was solved by social mechanisms. ("Hey, tell that loser to knock it off or AUP him off your system, wouldja?") DNS is old. We no longer think of DNS as a "phone book" style directory system, and we no longer assume that every reachable machine is cared for individually.