They make seatpost racks with pannier brackets. I have two of the plain type the OP shows, one with regular bolts for the clamp and one with a quick-release clamp, and one (regular bolts) with pannier brackets. Do not like any of them. They all twist too easily with any load, especially the pannier one, and the max load is very limited (like 20-25#). The only reason to use it is if I want to carry about a rack trunk's worth of stuff and not mess with or can't use a regular rack and p-clips; I did this a few years ago when my wife and I did a supported bike tour - other than water, I carried the gear for the day for both of us on my road bike with a rack trunk. My road bike now has a carbon seat post, so any seat post clamp is out unless I switch posts. Tarwheel's suggestion looks very nice and is what I would look into if I wanted to spend the $$.
A potential problem with seat post racks (and maybe a Carradice or similar) is if you have a small bike frame and not much exposed seat post - there may not be enough vertical space for the rack or bag to clear the rear wheel.
For a commuter, (or touring on a road bike, for that matter), I would get a consider a real rack (lighter, stronger, and cheaper) and p-clips, or a rack that uses the brake attachment, rather than a seat-post mount. The exception may be for carbon frames where compressing the seat stays is a bad idea. One of the guides on the trip my wife and I took had a very nice (Topeak?) beam rack and trunk combo for her also very nice bike she was riding.