Well, I would assume her resistance would be a pretty important stat for her, but if you're not really worried about that, then I think you should save your orbs. If you're worried about IVs, then there's no guarantee that you'll get one that's better than your Delthea's, let alone getting one at all.
Delthea vs Linde
So a couple of days ago before the New Power Banner I drew a +ATK -RES Delthea, so is it worth the Orbs to draw for Linde? (keeping in mind that Delthea will probably be my main blue mage)
My Delthea is much better than my Linde. They are very similar and neither one has or needs any very unique skills. There is no guarantee that you will get Linde and if you are pulling for merges for either of them, you are splitting the blue orbs for the both of them plus everybody else. I would save my orbs.
I have both.
Delthea gives +6 att to adjacent melee fighters. I have her set up with double drive attack so she can end giving +12 attack.
Linde's new refined weapon is pretty great. Pair her with a mage or healer and she gets Att/Spd +5 when within two spaces. This helps her hit HARD. Also, I've currently got her set up with Fortify Res and Hone Speed, so she works as a pretty good buffer for mages/healers (+6 att, +4 res, +4 spd).
I don't think Linde is "necessary", but she is pretty good. It's up to you.