Convention is a long time ago. UK vehicles without beam deflectors are legal in UK but do not recommend you use them unmodified in Europe, and I'm sure other examples could be found e.g. speed camera detectors or indicators may be legal in UK but not in France.
For the very small price of the stickers I fit them and remove a possible reason for the gendarmes to stop me, when they might well 'discover' more serious infringements if the are in that frame of mind
My French is passable to good but I really don't fancy testing it by arguing at the roadside.