I generally find that having a vamp scepter combined with IE gives me more than enough lifesteal to last through the mid-game, and it isn't until the late-game when you need to keep up with the enemy team's DPS that more and more lifesteal from BT becomes important.I don't disagree that IE is an excellent item but I personally hate building BT late-game. The full 100 AD and 20 Life Steal is great for Early-Mid where outlasting the fight is crucial. As I see it an early Last Whisper is like a Death Cap for ranged AD. The 46% Armor Pen cuts their defense into almost half. At that point in the game bruisers and tanks are between 100-150 defense. So averaging this 125 or after pen ~ 60 Armor or ~30% damage increase.
Some of my numbers may be exaggerated.. maybe not.
I also feel like Last Whisper as a first, second, and occasionally even a third item is outclassed by a lot of other options, unless the entire enemy team has managed to stack a lot of armor by the 26 minute mark. IE+PD is usually core, and for good reason.