good questions. Generaly its a one time thing to get them off to a great start in cool weather. Honey is not good brood food, light syrup is actually better so the goal is to provide a ton of it right in the brood area even when its to cold or you have no real active foragers. Even with a feeder the nurse bees wait for someone to go get it from the feeders. with it right on the comb the move quicker. after that first brood cycle your hive is in better shape, usually the weather would be better(not this year) and your balance of foragers to nurse bees in line.
I don't conssider them for dearths. just starups, and fall. I take all the honey in the fall, so several deeps frames of syrup will be put back. (first time doing this)
As for weight, didn't test that, but there full, frames are probably 6-7 lbs each