Think you guys are massively overstating just how many spells go through bkb.
It's not quantity, it's quality. Lets say you are up against this team:
Enchantress
Lina
Clockwerk
Vengeful Spirit
Faceless Void
You would probably still buy a BKB against this team, but yet the item isn't doing you that much good at the same time. Lets look at what BKB will take care of for you, I've bolded the things that really are a big deal here:
Enchantress
-Untouchable
-Enchant Slow
Lina*
-Light Strike Array (a stun, probably what you are buying BKB for)
-Dragon Slave (damage, but worth noting)
*For arguments sake, lets assume she was mid and got a quick Aghs.
Clockwerk
-Cogs
-Battery Assault
-Rocket Flare
Vengeful Spirit
-Magic Missile
Faceless Void
-Time Walk slow
Now here's all the stuff that does go through BKB, the bolded stuff is the most dangerous generally.
Enchantress
-Impetus
Lina*
-Laguna Blade
*For arguments sake, lets assume she was mid and got a quick Aghs.
Clockwerk
-Hookshot
Vengeful Spirit
-Nether Swap
Faceless Void
-Chronosphere
That's a lot of shit that BKB is doing you no good against at the same time. Obviously BKB shouldn't be a free pass to say "fuck positioning", go ham, but it really doesn't give you enough vs this team.