If you can reliably spend NX for rebirths, why not? Advanced magic packs more of a punch (even at Novice rank) compared to their bolt counterparts, and can be useful with a party in later dungeons as long as you have enough MP/potions.
It's really up to personal preference though, but seeing as you've called spending AP on Lightning a "waste," you probably don't have to get it at the moment.
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