Luke Skywalker journey

Specific Omegas and Zetas are incredibly useful for that event. Farmboy's Bullseye ability, for instance, should be Omega'd. It's very hard to crit in this event, but that's not a bad thing since with the Omega you dispel buffs when you don't crit using Bullseye. 

So if you have the Omega, you can wait until someone taunts, then dispel it using Farmboy, and that allows you to focus damage on the enemy attackers. That strategy is important on at least 2 tiers, though I played the event so long ago I don't remember which ones. (Replaying it now would be useless, all my toons are relic and would one-shot enemies so I wouldn't have a chance to try different strategies.)

Meanwhile the STHan zeta is important for protection recovery, allowing you to survive much longer than otherwise. If you can mod him for Protection and Potency, that also helps. 

Princess Leia needs an Omega on her special Rebel Tactics so that she can stay under stealth 100% of the time. The zeta on Against All Odds is very useful if you can afford it, but it's not necessary and if you're going after CLS then you have much more important uses for those zeta mats, so I wouldn't spend it on Leia.