They want an established islamic state, defeating christians, is the goal.
Destroying Mecca and Medina would deny any kind of capital for an established islamic state, and since Mohammad basically stayed in the middle east, there wouldn't be to many options for them to turn to (plus it would also make it impossible for the same islamic terrorists to carry out certain practises in accordance to their religion).
I also believe this is why, the terrorists have never tried to blow up or bomb any of their own holy sites and then blame the jewish people or the United States, while in the middle east, the arab population would certainly believe any conspracy theory and security at mecca is quite lax (at best), the fact that the terrorists have not done it, leads me to believe that they view their own religion first, before anything else.
al Qaiada blew up a holy shrine in Baghdad which ignited the desired civil war between the Sunnis and Shia...
I'm reading Lawrence Wright's "The Looming Tower" right now. I was not aware until I started reading this that Osama bin Laden's father built The Grand Mosque in Mecca. Yeah, the big one with the black stone in it. He also built the big mosque in Medina. Interesting thing, that.
He took a Caribbean cruise once and spend one winter in Australia.