I think there's a worry about some Al Queda / ISIS types taking over and causing havoc with the Saudi trillions. The Saudis may be undemocratic but at least they don't cause too much trouble.
You can read this article, or google around for better ones:
http://www.huffingtonpost.com/dr-yousaf-butt-/saudi-wahhabis...
It's a very complicated relationship, I think the best long term outcome for the region and the world is a massive reduction in the importance of oil and the Middle East ceases to be important strategically.