Most people worth half their weight in salt time summoner spells themselves in game anyway. The only ultimate with a cooldown longer than 3 minutes that I can think of is Karthus, and if you missed when that was, you deserve to not be prepared for it.
Zac and Anivia Egg would be worth timing, but they have static cooldowns so if you know the cooldown time of it you know when it's up. Aatrox might have value, but it's visibly easy to tell when it is or isn't up as well.
So really there's nothing of value that you could learn from having someone tell you stuff in spectator mode that you wouldn't have access to any other way.