From InstanceDiagram: I've wondered for a long time why people make such a big deal of class diagrams and tend to ignore instance diagrams. -- RalphJohnson 

Indeed. We use very simple instance diagrams (and CollaborationDiagrams?) sometimes to record the outcome of CRC sessions. Sometimes we then summarize a bunch of instance/collaboration diagrams onto class diagrams, though it's not clear what benefit we get from this beyond ScratchingTheItchOfHabit. -- KeithBraithwaite