If not for the durability mechanic, spamming Arcfire would be a win/win in so many situations.
It's like PK Fire, but it ALWAYS activates, no matter what it touches. For example, if your opponent is holding the ledge, without tome depreciation you could keep spamming it, and force them to jump from the ledge to get back. You can even throw it on the ground to deny ground approach, and even if your opponent will block it, the hitstun will be such a huge detriment on their aggression. Since it is on a downward arc, even on reflection it is pretty safe as long as they are not too close.
I think the durability mechanic allows the designer to have a bunch of moves that would be otherwise big no-nos in game design, since suddenly a limited resource is involved. Going by this line of thinking, I wouldn't be surprised if Elwind doesn't put you into free-fall. It's not like Robin can just stall off-stage forever; there is probably only enough juice on a fresh tome for three uses. Also, healing, even if it's attached to an attack, is another thing that is very difficult to balance if not for the fact Robin can only cast Nosferatu so often.