I think the Thunder would be a worse team if you swapped Westbrook for a passer like Rondo.
I don't agree with this.
Westbrook being a greater scoring threat may open up the defense but it also requires a willing passer. Westbrook isn't unwilling but he isn't remotely on Rondo's level in that regard.
I think Rondo's ability to run plays, plays that can work to get Durant easier, open shots would just make him much more efficient (than he already is). I think it would also serve to get more consistent offense from the other players on the team. Ibaka is good enough to be the 'number 2' scoring option with Rondo being number 3.
Basically, what you'd lose in individual scoring would be made up for, and then some, with improved efficiency of the overall offense, and most importantly, your best player.
Interestingly, an efficient offense can have a positive effect on transition defense as well.