In 1979, the Wahabbis took over and threatened to destroy the Kaba at Mecca -- so it's not an alien concept for them to try again. The Sauds wound up killing almost 70 of them, as I recall.
The Mahdi (the larger-than-Christ figure in Islam) is supposed to appear in Saudi Arabia, which fits in nicely with the extremists plan to take over SA. It pretty much all fits together nicely -- a nice PR spash for the murdering savages in the minds of the deluded and brainwashed populations of the Middle East, if they can pull it off.
As much as we might detest the Saudis, a takeover of SA by terrorists would be a disaster not only with regard to the world oil supply, but in giving terrorists the holiest place in Islam from which to wage war, so that any retaliation by surrounding countries or by the U.S. would be seen as a religious war.