In the Episode III radio show I heard something interesting...
Mace Windu said to Obi-Wan that the reason Obi was the one to go and defeat Grevious was because Obi's style was different than Mace's.
The problem for me is that Mace nearly killed Grevious each time he face him!

For instance, on some planet in the Clone Wars graphic novel, Mace brought a chunk of starfighter down on the General's head! Also in the Clone Wars Volume II, Mace smashed the cyborg's chest plates!

Why would Mace send Obi when he had been so successful against Grevious? Did he doubt he could kill him or did he think that Obi might be able to capture Grevious without killing him? Or did he have some other plan for sending Obi (by himself) to find and subdue Grevious?