A monopod is useful pretty much only for the prone, requires tedious manipulation between shots, and is an impediment for multiple and moving targets.
A decent bag does all of the above, as well as offer stability from "field" positions (shooting off of stuff that you encounter in the real world).
Dedicated rear-bags are less overall useful than something like a Game Changer, but they can be smaller and lighter.
For competition I use a shmedium heavy game changer, but if weight is a primary concern, I use the pint-sized sticky lightweight game changer. I have used it successfully in numerous hunts as a rear-bag as well as shooting off of fence-posts, truck beds, cattle gates, rocks, window sills, and various other positions. The heavy is definitely *more* stable, but the sticky pint sized is better than just about any other option that isn't a similar design intent bag.