I commuted with panniers for a couple of years before switching to a trunk bag. I realized I was carrying stuff just because I could. The trunk bag has plenty of room to carry what I need but doesn't tempt me to throw in a bunch of stuff "just in case" (like the jacket I carried for a year but never wore). I actually noticed a difference when fighting headwinds not having the panniers. Seemed to be a little less drag. But either panniers or trunk bags are superior to backpacks IMHO.