I think it is a give and take that varies between the individual. I see many people with a John Deere hat or gloves or belt buckle that never owned a tractor. Same thing to me.
I live and work within an hour of 5 dealers. I didn't have any experince in Harley Davidson's when I bought my bike and I had not found HDTimeline at that point. A dealer makes it convenient to physically inspect things like jackets, chaps, boots, saddlebags, helmets etc. So, from that perspective they fill a niche, for those that what to try things on and look around. Kind of like kicking the tires, if you will.
Right or wrong, I think the dealer and the HD brand give a starting point for newbies like me. Again, at that point, I didn't have a "riding style" so I didn't know what I didn't like. I felt better at least being able to hold things in my hands. I guess I am speaking more of what I would consider "gear". The only items I collect are firearms and I would consider them on the same plane as an HD coffee mug, but that is just me. I would think that the "knick-knack" buying group is probably 50% HD owners and 50% family/friends of HD owners that want to get a meaningful gift on holidays or birthdays and the like.
I agree with you that the bike is a big enough statement, but again if buying a shirt or a key chain or whatever makes you happy and comfortable: go for it!