I like the internal frame packs, but I can see why other's like the external ones. The little pockets for storing items that get swallowed in a single Duluth style pack is a nice convenience.

 I have put add on side pockets to both my main pack and food pack.
I carry my stove fuel in them, ( no fuel leaks in main bag worries) and any other item that I might need in a hurry/emergency. They hold a surprising amount of stuff, and attach too any pack that has dual side compression straps.

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 They don't seem too snag up on portages/bushwhacks as the main pack is narrower than my profile anyway. Same in canoe, they don't hang-up with their rounded profile.