I'd rather have a real pack and protect it with a duffle. Given the habits of airline baggage handlers, a lightweight pack would get ripped, etc. The exterior pockets and straps that make a pack usable would suffer too. I have a couple travel packs, and they are okay, but I've only used them as packs while walking shorter distances to get from a train station to a hotel for example. Ultimately, they end up being heavier luggage and poor packs.
Patagonia makes an UL messenger bag that would be great for a carry on and around town. I prefer a shoulder bag in the city as they are better on buses, subways, negotiating crowds, etc. Wearing a backpack on a crowded subway car leaves you wiping the nose of the person behind you and getting a lesson in cussing in the local language:)

