I don't see how center knurling could ever touch your shins. As someone mentioned, the knurling is only a few inches wide. My chins are scarred up from deads but that is from the knurling outside the smooth area, as I'm sure it is with everyone else
There are a few reasons I would prefer a smooth bar:
1) If I was doing olympic lifts. A smooth bar does less damage the skin in the around you collar bone area from catching cleans or doing front squats.
Also, once I scraped my nose with the knurling doing clean-grip power-snatches. But that could be because I am an idiot.
2) Smooth bars slide better on your thighs during barbell shrugs or the top few inches of a deadlift.
Not really great reasons to go with a smooth bar I guess. For some reason I like smooth bars though. Dunno why.