You don't know that McClintock would be a disaster any more than you know that Schwarzenegger will be the second coming of Ronald Reagan. Charisma is not the same thing as star power and political leadership and effectiveness (Calvin Cooldige, for example) can arise from very ordinary-appearing and plain-spoken men who rise on their principles to master the most difficult challenges.
Do you think the legislature is going to give McClinton even a NY minute? If an outsider like Arnold wins and wins big, they will have to give him some consideration. Someone who makes a living in front of a camera is goign to be more persuasive with the public than someone who cuts backroom deals.