Welcome to Librarium Online!
I've been playtesting a jetbike list recently. the full list is somewhere in the fourms. i've been using a lot of wolfpack type tactics, using my mobility to circle round units and bring a full army of firepower to bear. it's good, but i can't help feeling it could be better.
My jetbike seer council is currently 1 farseer and 7 warlocks. I mainly send them after tanks, monstrous creature, and to tarpit units for my shining spears to wallop them and them hit&run.
however, i've come unstuck slightly against the likes of assault terminators. i just can't generate the volume of wounds required.
now i know that doom would be a huge force multiplier for these sort of tactics, but to get doom and maintain fortune, i need spirit stones and doom, which is 50 points (IIRC). how much resiliance will losing one warlock from the council (making them 7 strong instead of cost me? this would enable me to re-roll wounds for 3-4 units at a time, so should significantly increase my killing power.
I'd say the gain is significantly greater then the loss. But try it out in a few test games.
The only reason I don't run doom is that pretty much everything I have wounds on a 2, so it's hard to get it to pay off. If you're running jetbikes and wounding on 3-4-5 all the time then it's definitely worth it.
i do have a fair few shurican cannons, but with no low ap stuff, i'm generating kills through high volume of wounds - which i've always maintained as the best way of doing things in a balanced army in case you run into big hordey armies. doom will only be useful if i can continue the wolf pack style tactics, but i'm begining to think more and more that it is such a force multiplier, i should make room for it.
I'm running the council, 3 jb squads and shining spears with an autarch. the spears are there as a big charging cavalry unit, not a unit to break the back of the enemy, and the council will only generate 4 or 5 wounds statistically on an meq squad on the charge, so they have to work in tandem and with effective shooting.
If you want to look at it from a mathematical perspective, then Dooming a unit will increase your wounds by 25% (4+ to hit assumed). Warlocks have 3 attacks on the charge and so you have 21 or 18 Doomed. Hitting on 4+ your looking at 10.5 or 9 Doomed hits and then 5.25 or 6.75 wounds. This is only one was of looking at it. Against high toughness and 5+ to wound it looks more like 3.5 or 5.
There are other considerations like: a)every Singing Spear you have in the unit will reduce Dooms effectiveness b)every Warlock you lose loses you a Destructor or shuri catapult shots c)more models=more get to combat d)More models=harder to hide from shooting.
I guess do as eris suggested and try it out. I would personally just keep the models the way they are. The extra few wounds is really not going to help that much against 10 Termies and a Character anyway, and once you lose the Farseer (I would target her since she's just as easy to kill with a hammer) there's fewer models to take the punishment.
Take doom. Its good, and proves useful in a number of ways. if it makes you kill just 2 space marines its already made its points back. Doom, Fortune, Guide and Wave serpents make an eldar army so devastating. Use them.
id drop 1 warlock for doom anyday, especially in a squad that size.
Eldar -APOC- 6500 Points
Chaos Daemons -Fate Crusher- 3500 Points
Grey knights - 1000 Points
I would ignore the assault terminators first and kill the humble rest of his army. Then I would shoot them with all I have. On their own the terminators will never see close combat against a jetbike army.