It would have too much burst, you could get multiple spells and autoattacks off in a short amount of time and just instanuke them. It's % current hp too, wouldn't be too useful on ad casters would need burst. It's a sustained damage item, only ad carries and bruisers do sustained damage, and ad carries would rather have BT.

BTRK is in sane on champions like draven and tristana who rely on crits/AS...

BTRK
Sword of divine
IE
is just a brutal on draven and tristana to the point they no longer require a PD. Tristana gains really good bust with BTRK and her e and w followed by a couple of AA's from sword of divine

BOTRK is actually a decent item for champions with onhit builds that include hurricane.

I build this after cleaver on Vi. The active is actually the main reason I take it, or well I take Bilgewater cutless initially at least because Vi often has that melee problem of being just to far away to finish a target. And well this is a much better upgrade then Hextech gunblade, plus as a bruiser with lots of armor pen I will often be targeting tanks and other targets on which the dmg is really worthwhile.

So ya it's a good item for Vi.

This may be a stupid question, but can't BOTRK, doing magic damage with its passive, become less valuable if the enemy team builds MR? If so, that 4% magic damage doesn't seem great.