Mine are just personal use, not looking to sell gold, or even make truckloads. I'll have one to farm skinning and one to gather herbs/mine. I'll get them to 615 so they can LFR (I think they're both around 610 atm so a couple heroic runs should do it). I'll just do those heroics and then LFR using Enyo rather than trying to do something afk. So ideally they'll be in 640+ gear, but not in a rush to get there. Kill faster, die less. They'll also do elite trapping for barn so want them to handle that better than atm. Doing the world bosses also makes sense if you notice a group filling quickly for them.
For me, the small amount of additional effort to gear them up is worth it. Plus, I figure it never hurts on a personal account to have your 'farming' chars do other stuff sometimes.
I would imagine larger scale farming operations would aim for less gear, that they can get automated.