I found them generally useless. That said, I have them all at L9, so everything I say is based on that.
Lightning was worth it in the first… 4 or 4? levels (Note: I mean the first four battles not skill points), then it got too expensive because I could do more damage by summoning a new unit. Now that I’m L40 doing endless/last mission, 200hp from an enemy with 92k isn’t enough for me to bother with giving myself carpal tunnel.
Frenzy… Useful, though I generally didn’t have to use it.
Healing – I don’t think I ever used it, either my healers kept up just fine, or the unit got one-shotted.
Crystal – Never did enough damage to be worth using. I have pulled off perhaps 5 last-second saves with it, and I have a mostly L40 party with 90 gold stars.
Pushback – Useful, but the cooldown is so long that, generally I would pushback + frenzy + crystal + lightning as a last ditch effort only, it being more efficient to summon more units instead.
Dragonfire – I use it all the time for the pretty fire, but it doesn’t do any appreciable damage to the revenant I fight, and never did really from the moment I got it.
If the spells scaled both with Azra’s level, the level of the enemy, and the level of the skill, they might stay relevant… As is, only Frenzy is ever really worth it. Azra has an attack value even if she doesn’t actually attack, this could be part of the scaling factor.