Nanaly is probably the most OP unit currently in our version of the game. But that doesn't mean she is necessary, but yes she certainly helps. If it wasn't for her, I'd not even bat an eye to the new account. But the fact she is there makes it a tough call.

Personally, I've cleared and 3 starred everything available before I had got her. And even now that I do have her, I've not hardly even made use of her since my strategies don't account for her higher unit cost. But I suspect she will be quite useful in future events. She can be used as a first deploy unit, and by herself can take a map with a difficult beginning and turn it into an easy victory.

I'd suggest playing with her some, and decide for yourself if she is worth restarting the game with.