User blog comment:Rapacious/Rapacious's sexy guide to understanding ranged AD builds/@comment-4199297-20120203142959/@comment-3238314-20120206210932

But the problem is this: Infinity Edge and a vamp scepter will cost you as much as the bloodthirster and a zeal. You'd have to farm 6000 gold (berserker's, zeal, vamp scepter, infinity edge) before the team fights start for this to be feasable, and this is a pubstomp, not something competitive, so it's a bit unlikely to happen. I don't think Damage scaling is that big of a changer in wether the champion should get bloodthirster or not. Infinity edge gives 80 AD, which is 20 more than 0 stacks BT, and 20 less than full stacks BT.

Besides, we're talking about Ranged carries, not AD casters. You are dependant on your auto attack most of all, even with great scalings, so infinity edge should always win. It's just that BT provides good damage with the added extra survivability in the early game.