MarkersSymbolizer, as I understand it, is currently test code.

I tried adding a <MarkersSymbolizer/> rule to a shapefile dataset and it produced blank output.

As I understand it MarkersSymbolizer is where vector symbols like directional arrows (for things like one-way streets) will live.

OSM currently renders one-way street arrows with Mapnik using a PNG file:

   <LinePatternSymbolizer file="/home/mapnik/mapnik/symbols/arrow.png" type="png" width="74" height="8" />