For starters not all all ap benefit from this item, in fact that ones that benefit more are the already popular, but let's say for a second that it benefit all ap equally, those ap are still overshadowed by the already popular ap. So their viability didn't change.

From what I've heard, Zyra's plants also proc the passive on this item.

I could see this, her buff control, and her ability to do true dmg after death, making her and attractive choice on TT.

Damage type does not matter. Casting spells is what triggers it. Alistar's trample is the odd exception to the rule that passives do not proc spellvamp/rylais/bft.

That, and Twitch's passive is on-hit, and to my knowledge, on hits don't trigger bft.

Nope. Chogath's might, because they made his E an exception for rylais/spell vamp.