The simulation was a huge resource drain in SC4. So I can see how offloading it would be a speed up. That said, if it's such a drain on today's insanely powerful desktop computers, they must have an insane backing server infrastructure in place to support it. I've got 50W worth of processing power available for the sim engine (investing half of my CPUs; still far more power than I had for SC4), are they going to put up 50W of processing power while I'm playing?
I can't imagine they've really got that. Either GlassBox is simplified, or made more efficient, or important parts of it are run locally. Either way, using it as a reason to have it online-only is a sham. It's just DRM, the same kind as Assassins Creed's online-only protection.
I'll buy it anyway. I wouldn't even care about the DRM, it's everything else that I'm hearing that scares me.