Well, I think Jamison helped create the definition. Jeff Green would be an example. A player thats just a little too undersized both height wise and strength wise to be considered a PF on a permanent basis, and just a little too slow, and lacks a good enough outside game to be considered a SF on a permanent basis. So basicly its a player that has skills, but just doesn't have a true position due to some of the reasons I listed.
Sort of the same way the term combo guard was created. These guys have always been there, we just never labeled them before.