Why should I upgrade raging spirits?

Hi there,

I'm following a minion infernalist build for my witch, and it wants me to use raging spirits. I have raging spirits at lvl 8 and when using my level 10 spirit gem, there are no noted upgrades to leveling it to level 10. The mana cost and intel requirements do go up however. What's the point of upgrading it? Is this a glitch?