For people who mostly play "trad" games, the immediately obvious use for Microscope is to spend a session or three creating a setting and history, both GM and players together, and then using that setting as the backdrop for 'ordinary' games of D&D, RuneQuest, Call of Cthulu, or whatever. This sort of thing is apparently already going on, all over the place, and lends itself to 'troupe play' with rotating GM-ships and adventuring parties, a la Ars Magica.
Now I learn that it's being taken a step further and people are creating dungeon histories using Microscope. To which I can only say, "Cool, or what?" ("What" is the wrong answer. "Cool" is the correct one.) I wouldn't want to do it all the time, because let's face it, discovering the history of a dungeon (that the DM has come up with by himself, in a darkened room) is part of the joy of dungeoneering Old School Style. But certainly I think there's room for it in the kind of 'troupe play' I mentioned above. This would see the dungeon as being, basically, a shared world, which people could GM and run games in on a casual basis.
The USP, of course, is that you know the broad sweep of the dungeon's history, so you could have different adventuring parties exploring it at different points in the timeline. You could even have groups of adventurers discovering things the aftermath of previous forays by other adventuring groups from decades or centuries earlier. To which I also can only say, "Cool, or what?"