Eh, it depends. Faster bike traffic shouldn't really be in such close proximity to pedestrians. On a lot of city streets, a fast bike is way closer to the speed of a car than to a pedestrian. City centers, especially non arterials, I'd say they should be in the street if there's no path. I'm not particularly fast at just under 30kph and it's rare for traffic to be much faster downtown here, especially non arterials I'm often passing them. I think that's generally too fast to safely ride on a sidewalk, but a safe speed there would make cycling not very practical for me.

Which I can assure, unfortunately, no cycle paths is often the case in my part of eastern/central Europe