"The friends we've chosen in the region also don't help us"
The recent OPEC decision declining to reduce production in the face of falling oil prices, thus screwing over Russia hard, means that Saudi Arabia is most certainly a useful US ally.
All allies are useful, even backstabbing frenemies like Saudi Arabia. But Saudi Arabia (or more specifically, the ruling regime in Saudi Arabia) needs us more than we need them these days.
I mean, I realize the prime reason they fund Islamist terror groups is to cause regional instability that drives up the price of Saudi oil...
That's actually a terrible reason to fund armed groups. Al Qaeda's original mission was to replace the Saudi monarchy; there were many attacks in Saudi Arabia after 9/11, they weren't talked about much. So the Saudi monarchy is not interested in helping those kinds of groups.
Armed groups are often funded by wealthy individuals for a variety of reasons (Osama bin Laden being a famous example). Armed groups funded by the Saudi monarchy are in other countries, fighting the regime's enemies by proxy (eg. the Syrian and Iranian governments).
The recent OPEC decision declining to reduce production in the face of falling oil prices, thus screwing over Russia hard, means that Saudi Arabia is most certainly a useful US ally.