A lot depends on how long your commute is. If you have a 2 mile commute, weight won't be much of an issue. If your commute is 20 miles with hills, you'll probably be more concerned about weight. I also carry stuff in my panniers "just in case" (my jacket, headband and cold-weather gloves spent all of last winter in my right pannier - actually, they're still there) but I've only got a 10 mile R/T commute. I'd probably move to a trunk bag if I was doing serious distance just so I'd have to keep my load light.