I think it really depends on the individual fish and how much pressure its seen. I have watched schools of fish split around a float as it drifts overhead. Drennans make a hug difference here.
Depth of water I would assume makes a difference too; fFish holding in deep water must feel more secure and there for less bothered by stuff floating on the surface. Slower flows where the fish can evaluated everything (including your float) probably are better suited to drennans.
Fishing is all about presentation (unless yur flossing
). I would say stack EVERY little advantage in your favour and your should have much better results!
Drennans are one such advantage. While not critical in every situation, they make a huge difference in some so why not use that egde to your benefits. I spend $20 on a tank of gas to get to the vedder, another $7 fora float that may help maximize my chances...might as well.