This makes sense. Caps could only buy you so much stuff from npc vendors but the good stuff you'll want will be drops from enemies or crafted. Also, if you look at it in terms of lore, I believe we are being looked at as some of the first people out of the vaults and its oy 25 years after the bombs fell. Hence why we are the only humans around and how has caps as a currency already been established? Or am I missing something and they've always been currency while living in a vault?

Exploits will always happen in the beginning of any online game. But I like you take on it that caps wont be that important but maybe just neccessary to get a few crafting materials from vendors incase you dont feel like spending the time searching for them.