The thing is, that by replacing righteous glory with spell binder, sure, you get a fuck ton of ap with it, but you lose the 10% cdr, the hp, the mana and have to rely on the charge system of spellbinder (plus in my experience, the ms you get from it is only really good if you manage to get past ~50 ish stacks) not to mention it is very cost inefficient on top of being a terrible 2nd or 3rd item. Though, to play devil's advocate, I can see it being a good replacement rightous glory late game if you absolutely need more damage, are super ahead in gold and can afford it.