Cultivate vs Flare of Cultivation

So I think I need some explanation on this one. I've been playing magic off and on for a few years now but I'm coming off of a break and stumbled upon the "Flare" cycle of cards from MH3.

Can someone explain to me why you wouldn't always run \[\[Flare of Cultivation\]\] over \[\[Cultivate\]\] if you're playing a Mono Green deck?

I could maybe see the argument if it's not Mono Green that you don't want to have to pay the 2 Green pips versus the 1 Green Pip, but I feel like having the opportunity to play a spell for free is always better.