Celestial Oposition's power and utility lies on the fact it extends all buffs on everyone so its power and effectiveness are based on how many buffs you're extending. Of course that extending an expanded card, dirunal regens and Collective Unconscious' regen are all obvious choices but the fact that CO cd lines up with Luminiferous Aether means that you'll always have something to extend if you time both together. This doesn't mean you should only use CO with LA, you can use other times but if you manage to extend LA together with other buffs (cards, snastry, regens or even lightspeed) means you'll be taking a better use of CO's power, even if 5s feel pretty weak.

CO on Lightspeed isn't bad because it reduces mana cost, using only for Lightspeed however, that is pointless indeed.
Something that is nice to point out: you can only extend Synasty with CO and only if you extend the buff on the Synastry target together with your own Synastry buff.