KG was our best overall player, the Captain was our best offensive player.
Rondo had a TS% of .483 that season (44.8% FG%), and turned the ball over on an estimated 23% of possessions. He had the worst offensive rating of any of the starters, and he only averaged 12 points per game.
Rondo was fantastic in the playoffs. He was right there with KG. In terms of his regular season, though, it was sub-par by his standards, and by the standards set by KG and PP. It's weird to say somebody who led the league in assists had a down year, but 2012 was not Rondo at his peak. Plus, he missed 20% of the season due to injuries.
I think people might be misremembering Rondo's season a bit, due to his great playoff run. He was our third best regular season player, and it wasn't particularly close. (Rondo was well behind Pierce and KG in terms of PER, win shares, win shares per minute, offensive rating, point differential while on the floor, etc.)