I mean especially on Vi because she takes no damage from the creeps due to her passive shield (after a few levels in E they wont get to break the shield). This lets you regen as much hp as you want, letting you stay healthy for ganking, possible counter-ganks by enemy and so on. And yes, her abilities are decently mana-abusive, not much, at all. But it's there, and you require mana to clear camps faster. Mana and health regen are never a negative stat. It's not something to stack, but it is never wasted as staying out of the base for as long as possible is the absolute best.
HP5 scales late-game the more durable you get. It's fairly obvious. you have health, armor, mr = you stay in the fight longer, you stay alive longer. The longer you stay alive, the more effective HP5 gets?