I know its a great Passive - its kick a$$ seriously with 50% extra INT - its crazy damage.
But here's some of my flawed logic on why I do not use it:
1) Health globes - Yes the very fact that it need you to pickup health globes to stack GF. My logic is that I always try to leave health globes untouched, as for me they are for replenishing my HP in case of emergency. So if I am >50% I try my best to ignore those globes and leave if the a rainy day.
2) Range style - the other reason which is partly related to reason 1) is that I play range style. I let my pets tank and hold up the mobs while I spam from far. As a result the globes are all far from me and thus reason 1).
3) The duration for GF - its only 10 seconds. And due to reason 2 and 1. By the time I picked up the globes the wave of mobs have already all die and the time it trake to meet the other wave, I would have left with about say 4 seconds of GF - pretty pointless now.
4) The MP levels. Another main disadvantage of GF is due to the nature of the MP levels. Lower MP - I don't need GF for the extra damage, PtV is good enough for me. VQ also takes care of my mana. At higher MP, GF becomes useless as mobs dies slower and globes drops are slower.
Can anyone convince me otherwise to use GF for farming purpose? I know some othe builds do good with GF like the Final Gift, Life is Death, Circle of Life build but those are not my style I guess.