I once checked into a hotel that had just recovered from a power failure minutes before I arrived. They couldn't yet pull up reservations. They said the only reason they were able to give me a key immediately (without waiting for all the systems to come back up) was because I had my printed confirmation with me. (The machine that generated key cards had come up already.)

(This was a property in another hotel program, but I don't see why this couldn't have happened at an HHonors property.)

How likely this is to happen to me again (or to someone else), I don't know. But it adds to the reasons (such as rate disagreements) that I like to bring my reservation printout with me if at all possible. (Having said that, I belong to a number of different hotel programs, and I don't want to have to have a different procedure on this at HHonors than at other programs, so if it's useful to have the printed reservation in at least one program, I prefer to try to do it everywhere. And there are other hotel programs where for late evening check-ins, the reservation may not be showing online any more, so I wouldn't want to rely on being able to pull the reservation up after I get to the hotel, even if that may work at HHonors specfically, again, because I don't want to have to remember which hotel program it works at which hotel program it doesn't work at.)