Outside of the laning phase/early game, BS is pretty bad. The problem is that he doesn't do either of his jobs (ganking/carrying) relatively well. I've never seen a BS become a good carry unless he's been dominating for the majority of the game. Sure, any hero could potentially carry in such a situation, but the biggest reason why I said this for BS is because it exemplifies what he needs to carry - to be way ahead of everyone. If you give equal farm to BS and any other carry, he'd lose straight up against that carry. He just needs so much more gold to stand a chance, and that's not reasonable. You could give yourself a damage boost with the silence, but then you'd leave an intel hero with access to all her spells. BS's spells are good against a full intell team, but if he tries to gank someone who doesn't rely on spells (or a DPS hero), his spells are pretty worthless. Rupture can be a really good spell for ganking, but because it lacks any sort of stun (or mini-stun) it's pretty easy to counter, and it's also pretty shitty in team fights.
Bane is a good hero, but it's range, mana costs, and cooldown for spells makes it atrocious for the laning phase.