Balancing something so fundamental as manabases is incredibly tricky
Like infect, mill is loathed by many players, even though its really just as viable of a strategy as any other and often backfires (most decks benefit from having more cards in their graveyard)
Eldrazi typal strategies love this, as does anything that uses a lot of morph or disguise effects, as those are colorless creatures on the stack, too
For its part, Ivy Lane Denizen puts a +1/+1 counter on any friendly creature that enters the battlefield
It's a great tool for protecting creatures from certain types of removal, especially when you can toss one around at instant speed
Ultimate: Honestly, its difficult for me to imagine needing this much rental value