Is there any functional difference (not related to things like embedded diamond grit) between a smooth-surfaced knife (or honing) steel, and one that has visible ridges?   Would there be any reason to prefer one over the other?  Clarification:  the type of answer I am looking for actually addresses the effect (or lack thereof) of the ridges at the microscopic scale where honing takes place. 
Keep in mind that a knife steel or a honing steel does not actually sharpen the knife, it merely re-aligns the microscopic sawtooths/burrs created on the edge of the knife from cutting to reduce the drag.    The contact point of a ridged-steel is less blunt (smaller radius on the ridge vs the steel) and might deliver more concentrated force.   with a ridged steel you can rest the knife blade and run across two adjacent ridges (creating straight contact with two small surfaces) and reduce the risk of damage (by bending the burrs the other way).  a round-steel contact point would be more blunt.  (we've looked at the honed edges under a stereomicroscope).  In my experience the ridged steel works better at aligning the burrs (or sawtooths).  To create a new edge or actually sharpen the knife you'd use something like a whetstone or a strap, or even high-grit (>2000) sand paper.  By removing the burrs this way, you create a smooth (new) edge.  