Arkel overdesigns makes a great commuter bag that I use a lot. It's the tail rider one. I used to stuff 50 pounds worth of groceries in these things. As for dead weight, well, depends on what your goal is. I kickbox, so more work is calves + pannier weight + 1999 heavy specialized hardrock classic with fenders and panniers = thick shins and calves that hurt the opponent.

Then again, that isn't everyone's aim. When it comes to commuting though, the most important piece of gear other than fenders and a rack are lights and 1 set of water proof pants. Then I'm golden.