20% of damage dealt to monsters as mana probably isn't that great.
It really only helps those that can repeatedly spam their abilities for strong benefit (eg.
or
), can get a lot of value out one or two abilities (eg.
) or those that can use damaging abilities for no mana cost but use mana (
). It helps those that are already strong in the jungle.
That's why I like the mana regen portion of Runeglaive's passive so much. It gives mana sustain sufficient for the jungle on a need-by-need basis based on mana costs and cooldowns, and this passive doesn't do that nearly as well.
Is Runeglaive good for jungler diversity? Absolutely not, but it's not the fault of the mana regen portion of the passive. It just needs the sheen passive removed, a new passive that keeps the mana regen either the same or expanded to support other playstyles, and better stats to compensate for the loss of the Sheen passive.