Plant gathering is a truly great thing and I'll tell you why. If you want lots of different kinds of booze and are farming, you have to fiddle around with lots of farm plots, seasonal crop rotations and the like. Bleh. At the moment I have an entire soil layer dug out for my tree farm and as a by-product of keeping it stripped clean of plants I now have over twenty different types of booze in my fort. Nothing keeps dwarfs happy like a vast variety of booze, not to even mention the masterwork roasts, each one with different ingredients. So strip the land bare of plants, strip the caverns, build a tree farm and defoliate that too. Plants don't belong in the earth but in barrels, waiting to make your dwarfs ecstatic with variety. I don't even bother with farm plots anymore (although it is good to have some in place for emergencies).
I just wish there was a way to
permanently designate an area for plant gathering and/or tree chopping, so that new plants/trees would automatically get designated for harvesting. If that was possible I'd use this strategy too, but the hassle of redesignating once a season or so for years on end gets to me; the only reason I prefer farming is that you can set it up once and forget about it for the remainder of your fortress' life.
You just build huge farm plots, more than you need for any given crop (and considering the sheer volume of food farms produce, a 'huge' farm plot isn't very big) and assign them to a variety of crops. Your farmers will plant in whichever plot they happen to have the appropriate seeds available for, and just leave any excess plots unused, it's completely micromanagement free. If any other food profession could be set up to support itself like that, I'd be using it.