Blazinghand is right. It's enough to have just two separate warlocks knowing the same power, say, Protect/Jinx, and one of them would be able to use Protect while the other may attempt Jinx. To do the same thing with a Conclave you'll need the minimum of 4 warlocks in a unit. Conclave would be far more expensive (+70 points compared to 2 individual warlocks) and far more vulnerable.
However, in this case Conclave is not without advantages, which come from stratagems. Firstly, a 4-strong conclave can cast up to 2 powers, and if you put a Farseer beside them and use the "Seer Council" stratagem, the Conclave would get +1 to cast both
of its powers, while with two individual warlocks you can only ever get +1 for one of them. Secondly, with another stratagem a Conclave can boost range of its powers to 36". Lastly, Runes of Battle powers do not require LoS - which means that with some strategic positioning a Conclave can safely sit behind a LoS-blocker and debuff enemy units which are 36" away, all the while enjoying +1 to cast and likely staying out of the enemy's denial range.
It's probably not a tournament-winning tactic, but in a friendly game I believe it's quite viable. And it's super-fluffy for Ulthwe!