Well, its like tennis , your not gonna get alot better playing people LESS TALENTED than yourself. You need to play against someone of simular talent level...and there aren't many people in the same league as these two fellows. Kobe would never do this ...he lives day to day beliving heis the best and only MJ might have been better in his mind.
They maybe the two best players in the NBA , and are young enough to think and try to improve their game. They can push each other or exhaust each other and test their energy and condtioning.
So ...yes probally makes sence to see if you can improve and use the other guy as a measuring stick (someone of simular ) skill level to see how your doing.
Don't see them being on the same team for a long time.