Depends on the adc imo, some adcs like Ezreal have a spamable ability that benefits a lot from having shiv. --- Going to church doesn't make you a Christian any more than standing in a garage makes you a car.
If you're super fed IE+PD is still going to net you the most damage. BT+Shyv is more cost efficient and better on ADC's with slow wave clears. IE+Shyv for LOL lightning dmg --- http://i.imgur.com/fiZMB.jpg http://i.imgur.com/TeSHa.jpg http://i.imgur.com/rBBQG.jpg http://i.imgur.com/xUh6r.png
The reason why you're seeing so many pros building Shyv+BT is because of the current strategy.
Made famous by m5 and perfected by the Korean teams, balls to the wall aggression leading to early towers and early team phase is currently the strategies used in most tournaments.
This seems to be the best way to counter zzz farm matches that Clg employed allowing them to win games even though Wicked and Hotshot gets camped and loses lane terribly.
For this reason Shyv+BT is ideal because it allows you to do relevant damage in team fights early on. Where as if you were building IE+PD you probably wouldn't have either item finished.
For most of us mid level Elo or Normal match players if you're not taking early towers and have the chance to farm by all means IE+PD.
*what I do when i'm not sure*Try building vamp, bfsword, and zeal. This is a decent(not always best) core on most ADCs. From there gauge for yourself how you think the flow of the match is going and build accordingly.
Also like someone posted above, Some ADC's have great single target damage but very little AOE or wave clear, you might want to build Shyv on someone like Vayne. Especially if you find yourself constantly being pushed to tower by that bully Caitlyn. --- 0% Class 100% A**