Overlapping also makes it harder to properly talk to the group, which just finished the game. Ideally, you want to give them a chance to provide detailed feedback (9 out of 10 will love to do that) without a tiny shade of being in a hurry. Probably go through the room, maybe explaining some stuff, maybe doing some storytelling about the process of the room creation (incredibly appreciated by experienced players) etc. There just zero chance to do that if the next group is already sitting in the lobby - first guys will be feeling pushed out.
Aftermath chatter is also the most effective moment for any kind of up-sell - you can't convince the same group to play another of your rooms if you haven't even left yourself any time for that convincing.
The hurry-up mode also leads to an increased amount of human factored errors. Even if your gamemasters are following a detailed reset checklist from A to Z, it's easier to miss a step while in a haste. It's also harder to solve the problem of small items missing if you are short on time. Yes, small items like keys, rings, amulets etc are quite frequently lost, placed somewhere or stolen (occasionally, but still). You can easily take care of finding where-that-damn-key-is if you have 30-minute gaps, rather than with 10-15 minute ones. That's usually a situation you wish you had a no-reset room.
And yes, even if you have spares (you need to have spares copies of everything, trust me), finding them where they are storageg also take some precious time off the clock.