Is it useless??

Why do Summoners get the ability to have 3 cubics at once, when they only have 2 cubics they can summon?? The odds you will be in a group with another summoner and both mass summon and summon your own are so incredilbly slim. Is it just a waste of SP??

They're not that slim at all. I've had a mass phantom or wind on my ES plenty of times.

Agree. I party with other summoners quite often and have had 3 cubics over my head many times.

third cube mastery is helpfully when you have up your own and other masscubes form party members oO