That sounds like a huge waste of a slot to me. Buffing and summoning are mainly used pre-combat and for more difficult fights, debuffs are only used against casters and a few spell-casting monsters, and damage spells are mostly used to clear trash. Sure, in a few fights, like for example against Dragons you'll want all four types, but in 95% of all fights Nalia will just be standing at range shooting non-hitting arrows.

The exception would be if you run SCS, being the caster clusterfuck that it is, but I think you would've mentioned if you did.

Also, if you're so hellbent on having a third caster I'd recommend Jan above Nalia. Currently your only thief is a 1/3rd of your main character, meaning you'll barely cover the lock/trap part of it. Jan could max out detect illusion and/or pickpocket (so you can steal literally every item in the game from shopkeepers) as well as abuse the Mislead spell much better than Nalia. And he'd cover your HLAs better, being able to pick up the traps so your NPC could focus on some Whirlwind shenanigans. Anyway, he'd bring a whole lot more to your party than Nalia would.