The downside of folders is trying to figure out where things belong in the hierarchy, or trying to update that hierarchy to a new standard.

The downside of tagging is you still need to establish conventions to ensure things can be found again, but enforcement of your conventions is harder.

Exploring, learning, and using an unfamiliar folder hierarchy is easier than exploring, learning, and using an unfamiliar tagging methodology.

But manually searching for something in somebody else's tagged data is easier than manually searching for something in somebody else's folders.