The purpose is to open driving lanes. The team's offense has been most productive this year when Jones, Granger, Ford, Hansbrough, and even Rush have taken the ball to the rack and drawn fouls.
Roy has been up and down in his low post effectiveness - mostly down in recent games, because he can't get good position. He has fortunately been able to score in other ways. But in any case he is not the center of the offense, and shouldn't be. On the other hand, coming off the bench against second-line defenders might be a good way to use his skills. Which brings us to...
Regarding the lineup change: it's time to shake things up. The coaching staff can't pretend that things are working after the Charlotte game. Foster was one of the best players last year. Hibbert has made great strides as a defensive player, but he's not on a par with Foster yet, and the offense ran smoothest last year with Foster on the floor (some people will think I'm crazy, but I think I'm on solid ground here).
Remember that Hibbert started a lot of games last season, but usually played minimal minutes in the second half - the crucial issue is how many minutes a player plays, or perhaps, as Red Auerbach said, not who starts but who finishes.
I'm not worried about guys losing confidence and all that. If they're fragile, they probably shouldn't be on a team that aims to contend for a title in 2012. The league is for grown men, and championship teams are for players who can overcome adversity. If you're not getting the job done, be a man and go work on your game. It's a player's job to persuade the coaching staff that they need him on the floor.