Safety first, as others have said.

Sidewalks are a part of my commute for a short stretch where heavy traffic is necked down into 4 narrow 10-11 ft wide lanes through two close intersections (and there are heavy turn movements there too). There are usually no pedestrians, and I take it nice and slow and obey the signals. I ride the road and bike paths for the rest of my 5.5 mi commute. Road is safer and smoother than sidewalks, especially the older sidewalks in our town which are cracked up and nasty.

Since that short sidewalk stretch I use is halfway into my commute, I think of it as a little rest/breather, too.