Sorry I wasn't clear in my post, was typing that on and off for around 40 minutes while having lunch, doing burst time, watching the market, and attending an online meeting all at once. So let me redeem myself by explaining better this time.

In response to OP's "I need orbs for past event eidolons as well... My first event was Nandi, and did manage to MLB her as well as Thunder Lilim and Fleurety, but they all suck for a light team, which is what I have.". I assume Light team is his main focus, and he is planning on focusing on one element instead of a big spread which is good. He has Nandi, Fleurey, and T. Lilim as sub-eidolons which he can already use for stat-sticks. So in terms of stat-sticks from past events, Tiamat should be his main focus. She has the highest stats from orb shop, and her barrier is useful so grabbing her essentially covers 3 advantages. 1) Stat stick, 2) A useful barrier that won't be replaced anytime soon, and 3) A main eidolon for water.

With Vritra event coming up next, although having a Thunder team and eidolon would give him an easy time, he probably has already spent a lot more resources on his Light team which due to the time limit could essentially mean rushing resources into a Thunder team now might not provide him better results compared to continued investment in his Light team. This would negate the immediate need for grabbing a past-event Thunder hime. With the addition of Vritra to his eidolon grid, he would now have 6 Eidolons at MLB. That also reduces the need for grabbing other past-element Eidolons. If anything, getting Andramelech before Tiamat could also be a good option considering having a fire team would greatly benefit him for the next tower. She is also a decently strong stat-stick that would allow OP to replace T. Lilim once he gets both.

The reason other eidolons are not as worth considering, is because he might not have enough time to invest in a Thunder team to make it worthwhile for Vritra Reprint, or he might not have the need to use elements such as wind/water/dark for other events. He should however, save up the orbs for when he needs a specific element. A mistake I made which he should avoid, is trying to get Eidolons of every element without considering whether or not he needs it. Not sure if you remember, but I did waste quite a bit of orbs for getting multiple LB1-2 eidolons of different elements, getting everything at an equal "spread" and end up not needing more than half of them. Either because it was a team I almost never ran, or I ended up getting better Eidolons(MLB Crom Crotch at +57 is the dumbest thing I could've done). I would have benefited more if I did not spend orbs for LB1 Icarus and LB2 Jack O'Lantern "when I did not need them at the time" because that delayed me getting my Tiamat(still 1 copy from MLB) or Amaru which would have given me a better Thunder Eidolon for the previous tower.

So what I meant to say was not that the eidolons are not worth it, but that it is better to save up on orbs for when he does need one instead of rushing.