I just don't understand what an attack on Saudi Arabia would do for Putin. The best case scenario is... Saudi Arabia's support for the rebels in Syria would end as soon as Russia bombs the shit out of the Saudis, and Assad would maintain his control over the country. But is it worth incurring the wrath of the United States, let alone most of the world, by attacking Saudi Arabia? Starting World War Three? Wouldn't the jihadis in the caucuses, under control by the Saudis, step up all terrorist activities in Checnya and the like? Fucking christ, "there's a war that's not going our way, let's start another war." I don't see the reason in Russia bombing Saudi Arabia. Then again, I don't see the reason in a lot of things going on.
The purpose is brinkmanship. Putin almost certainly won't bomb Saudi Arabia. Everyone knows he almost certainly won't bomb Saudi Arabia. But he doesn't want Obama to intervene directly against Assad. By showing himself as capable of considering an attack on Saudi Arabia, Putin raises the potential perceived cost to the US of becoming militarily involved (I say "militarily" because there's no doubt the US is already active on the covert level).