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.
It may be not be a valid reason for you, but Sim City ran significantly faster than Civilization V on my machine. To me, they both perform a massive number of simultaneous simulations, and Sim City's performance is near instantaneous.
That said, there's one huge factor none of the beta players can take into account when judging Sim City's performance: no one has been able to build massive, multiple-city metropolises, as play time was limited to hour-long sessions.