I also have some Harley T-shirts and I also find stupid to do free advertising for corporations. I just didn't debadge my car or bike because still people will know which one it is.
BMW helmets are manufactured by -if I'm not wrong- Schuberth. They have a very good reputation. The question that lingers in my mind is: why a biker would never think about buying a BMW or Schuberth helmet, but will buy one when he owns a BMW?
As it has been said, only a few Ducati riders wear a Ducati helmet, but the three Ducati helmets I've seen were worn by Ducati riders. Oddly enough, I see many Monster riders wearing Momo helmets. I guess that if you ride italian, you have to set yourself apart from the "crotch rocket" crowd, and it goes up to the helmet.
What about "Arlen Ness" gear? It seems good quality, but I would never wear gear that could relate me with the chopper crowd.
Motorcycling is probably the least logical activity -let's say "hobby"- ever performed by the human race.