The problem with KT in the starting lineup is that he takes pressure off the other team's defense--he allows them to play big if they want, because he's a magnet for the other team's weakest defender.
You sound like you think Thomas is another Bo Outlaw. He's not. He has a reliable elbow jumper -- in fact, he's probably a better mid-range shooter than Richardson -- and has averaged almost 13 points per game over the last four seasons, in a grind-it-out offense. On a game-by-game basis, Richardson outscored him by all of three and a half points last season. You don't think the advantages gained by a more versatile lineup can make up for that difference?
I must admit, however, that I'm shocked by how rarely Thomas gets to the line -- barely one attempt per game last season. Still, with the way the Suns' offense is designed, they wouldn't want him clogging the paint on the offensive end anyway.