... the line framed in blue (below) would shoot at the kurassiers while (or before, if the unit stops during the shooting) the whole unit continues on its way (red arrow)...
...instead of wasting time turning then shooting from its front line, giving the kurassiers time to turn and charge them (1) which would be devastating ?
Maybe such units were able to shoot in all directions (maybe with varying efficiency depending on speed/immobility and direction) :
...?
(Of course this question can go beyond the particular case of the Arquebusiers and P&S.)
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(1) btw imho AP (kurassiers) should be < AP (Arquebusiers).