Also one thing to keep in mind is that since you can only Promote a cannon minion, and cannon minions only spawn every 90/60 seconds, the cooldown is effectively 180 seconds even if you have Mastermind.
It's helped me a few times in lane when I've been pushed to tower against someone with way more sustain than me... promote and you have some time to go b, as well as getting the gold from stuff it kills even when you're not in lane.
Same with promoting before going to get blue or wraiths as mid.
"the babies cat crying sounds is turning me on like crazy." - Tommy2GoGo
How is it great? So what, you made one powerful minion. I know it was used in tournaments, but I cannot say that it was more than a minor annoyance. Usually they just plopped it down when they went B.I know it has it's uses, but people are greatly over-exaggerating it's use.
When you promote the cannon minion, you're free to roam the without as much worry of your lane getting shoved back.
Would it not be useful as a sort of 6-man split push late game? If you promoted a bot lane minion and let it do its thing, somebody is gonna have to go bot and kill it or give up a tower. If they send someone bot lane you have a free tower/baron. I'd still rather have another spell but I don't think it's useless.
The thing is, it has its uses, it just doesn't quite fit the current meta when there are other spells that can be more useful. Promote has to be used on a nearby siege minion, whereas flash, ignite, teleport, exhaust, etc. can be used almost everywhere you'd ever be. Promote is worthless in a teamfight in jungle or without a nearby siege minion. It's just too conditional to be picked up, it only really fits in specific team comps with good coordination.