"I suspect that the Leftists see the Muslims as useful idiot allies in their war against the West but think that the Left will always be in control of the situation."
I think you hit the nail on the head. The Left are perfectly comfortable embracing fanatics as long as they are useful and they think there is no threat to their own power. When the recognition that radical Islam is in fact a grave threat to their own ideology finally hits home, things will change. It appears they are already changing, but overall they will probably try to hang onto an obsolete picture of reality until the roof caves in on them.
Frankenstein's monster comes to mind. Some European leftists realize the monster in their midst -- the late Theo Van Gogh was one.