Because of the chance at getting an awesome roll. It made every drop feel unique. I loved getting the auto rifles with the perk that made them shoot slower but with more impact when you were ads. IB guns also had random rolls. I don't know if you mean the vendor's roll for IB were perfect, but a perfect roll is subjective and the best roll for one situation might not be the best roll for another situation. Even if you dismantled most weapons you get, the chance of getting an awesome roll made it exciting. The guns were at least worth looking at and seeing if you should keep them. In D2, I just dismantle everything that won't put my light up.