Hi,
at first I think it would help with aggro, if you get those 3 rubies, but I can't say if this will help in any situation. (Never leveled a summoner with the actual skilltree setting)
But mana problems at leveling is almost impossible with a leveling build, look on the 3 rubies "BLOOD TAP" in first grid

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DARK GIFT is nice yeah, but "only 3 rubies"... if you play summoner there are several rubies which are great, but you are out of rubies soon, if you want to take them all.
Look at the shield rubies for example of very nice ones. Sure the stamina buff is useful, but often there are more useful rubies, than those (but I don't say that they are useless).
TOXIC WEAKNESS works just like this:
Summoner inflicts an enemy with toxic weakness debuff, if he uses howl of death or vampirism/life tap now on this enemy the buff will disappear and the dmg of the used skill will increase by 50% on this enemy (Volatile Infection is not boosted! The ruby is wrong described or wrong coded)
Hope that was a bit helpful