_node_revision_access() hides the revisions tab if there's only one revision of a node. pwolanin mentioned at Drupalcon that this had been flagged as confusing (not sure who by so this is a bit anecdotal but it doesn't surprise me) - sometimes the tab is there, sometimes it's not. The page works fine if there's only one revision - just lists a single revision, and that way you know that revisions are enabled / that you have access to them etc.
It also saves us a COUNT() query on the node table when viewing nodes.