One reason we didn't ship a "Who's bookmarked this" default view is because we didn't have a way to put a tab on certain nodes only (Namely, nodes to which a flag applies).
The problem continues
Views 2 somewhat solves this by providing a "Node" validator, which validates only nodes of certain types.
But this isn't yet a satisfying solution: the node-types specified for this validator can go out of sync with the node-types specified in the flag settings. Examples:
1. The user edits a default view. He thereby creates a copy of it. He then adds node types in the flag's settings. These new node types aren't included in those of the "Node" validator.
2. A user "clones" a default view we provide, because he thinks it's a good starting point. He changes the flag in the relationship. But he doesn't change the node-types specified for the "Node" validator (either because he forgets or because he don't know too much about Views).
The patch solves this by defining a "Flaggable node" (and "Flaggable user", and whatever) validator.
I'm attaching a screenshot.
The screenshot shows that hidden here are _three_ related validators:
1. Whether an object is flaggable.
2. Whether an object is flagged.
3. Whether an object is flagged by the current user.