One bag, two jobs done well. And a small question about why I like it so much.
I recently bought the Knack Pack Small 2 and I've been using it consistently for commuting (when I have to commute, that is, because I work a lot from home). And as it expands for short one or two day trips, it's a perfect bag doing two jobs. That is the practical reason I bought it, and the practical reason holds. It is genuinely good at both.
That's why I like it. I know it's ready and capable for those two situations.
I think it is the feeling of being ready. The bag flexes to fit whatever the day turns out to be, the normal commute or the sudden short trip, and when it is packed and ready I feel prepared for all of it. That feeling is pleasant and a little addictive. It also has almost nothing to do with whether I am actually prepared for the day in front of me. The bag is always ready.