If it's bad form to argue with the premise of a thread starting post, it's probably even worse to argue with the wife of that original poster.
But here goes anyhow.
For all weather commuting, the knowledge that my work clothes are staying dry makes it worthwhile (for me, at least) to deal with the extra few ounces of the Ortliebs.
If your wife's load is so light that empty panniers are a significant fraction of the total load, she's not likely to be overbalanced by using a single pannier.
Have you/she considered a trunk pack?