Hmmmmm. Interesting question.
I have many bows, but only one that I ever use for 3D/field shoots. I've gone the other way from all you guys in that my 3D bow is far heavier than my hunting bows. My hunters are all either 57 or 58#, whereas my 3D bow is 72#
In the UK our field & 3D shoots are at much greater ranges than I'm led to believe they are in the US. I'd say that an average range shot for us is probably 35yds, with targets from 5yds all the way out to 80+ Several ofthe shoots I regularly attend will have at least 20 out of 40 targets at 50yds+ so for me greater cast is pretty important. The faster & therefore flatter my arrow flies, the better chance I have of hitting the looooong targets (furthest I've encountered so far was a 122yd, standing buffalo!)
It takes me a wile to mentally adjust to shooting the lighter hunting bows, but I feel that in a hunting situation, when the adrenaline's pumping & everything's jumpy, a lighter bow makes life easier by far! (plus, they're all take-downs. My 3D bow is a one-piece which makes travelling a real bugger!)