I'm mixed on this. I really dislike online DRM, but one of the greatest weaknesses of SimCity 4 was that the average home computer had no hope of simulating a large city.
This is still true today - while most computers could handle the graphics engine behind it, the simulation engine would still eat your machine for lunch.
Moving the city simulation server-side certainly has its justifications - it opens up the field to more players with weaker machines.
That being said, it's hard to imagine that, given the ginormous increase in CPU performance between SimCity 4 and now, that they couldn't have made a simulation engine fast enough (and optimized enough) to run offline.