well it is super-useful late game since it gives you an extra consumable slot... Wand of Speed or Orb of Defiance (+500 HP and Armor) makes it marginally better than Blood while also giving the 5 seconds of invulnerability (nice if you 1v1 and they sigil since you burn a quarter of a sigil's duration for 'free'). I dunno if you would "even if" use 10 scrolls. It's more like "when". I tend to buy 2 ports every second shop, 3 every one after that. And I'm /pretty sure/ I shop at least 4 times a game though not much more than that. Once at WS3 for Currency (no ports then), one time around level 8-9, once for catapults then defending portals or buying more sigils etc etc
It's nice if you are currency bitch since you are already gimped in terms of $ (though Blood is typically best for currency bitch).
That said, the only time I would seriously consider getting amulet is if there is a tower rook on my team. Having a port beginning with level 1 is nice. Also remember that it does help you maximize your levels since you only ever have to walk one way to base. Etc Etc.
That said, it is quite a weak favor item early game. Keep that in mind. Anyone with BotF will rape your face until the ganking/farming/teleporting manages to pay itself off.