If we can figure out a way to show percentage owned without it getting confused by rarity, I'd like to see that added as well.
And without that percentage being confused with the percentage of people rating the game
Adding more and more information isn't always a good thing, because game pages could get overly complex with numbers and statistics.
Just like the Black Perl said, we "only" have about 1000 collectors on RFG, that might not be enough for some games to give an accurate representation of rarity.