BT stacking

Just a question, but... sometimes I see people suggesting to build more than one Bloodthirster, or actually seeing it done in some past matches, etc., and I was wondering... is there any logic behind this? Both the Lifesteal passive (which is funny because that means you'd have more lifesteal with just the vamp scepter instead of upgrading a second BT) and the shield passive are Unique. All it gives you is the 80 AD... which kind of reminds me of the whole "stacking deathcaps" dilemma, where'd you just get more use out of buying a different needlessly rod (or in this case, BF Sword) item because the passive is unique and the raw stats are the same.
Why would you get more than one Bloodthirster? Or should I just ignore these suggestions?