I'm with you, for a mess bag to be practical, you'd have to be in a situation where you're unpacking/repacking the bag several times a day, but there are tons of other threads you can read about the benefits of mess bag over backpack. The funniest reason I've read was something like this:

"You can't fit a 30-pack of Milwaukee's Best in a backpack, brah!"

Of course most of the reasons you'll read are completely bogus because many of the respondents have never had a decent backpack. They say they're hot, but never had a pack with a suspended back, say they dig into their shoulders, but never had a bag with good straps ... etc.