I think a lot of people add games they have owned at some point in time, but I think that goes against the idea of the 'own' section of the game library.
Its supposed to tell users what you have, what you want. Afterall, adding every game you ever owned is really just a pissing contest at that point. Rather pointless.
Plus, some games would get duplicated as almost every game in my VC is one I owned as a kid. I think it should only contian games you currently own. Hell my Wii library alone would be doubled based on the games I've bought, beaten, and sold on Ebay.