never in my experience. we won plent enough without ever plaiting - we even came 4th at the BD winter champs without plaiting. every judge i know says plaiting doesn't influence them. you can be neat and tidy without plaiting and you can also look a mess even if you do plait.
Yeah I completely agree you can look just as smart unplaited. However I think overall impressions are important with Dressage, and as wrong as it is it can make a difference. your boy looks fab unplaited, but I think generally horses look correctly turned out for dressage plaited IMO.