Ard, I agree that carrying a full daypack will probably be more of a hindrance that help most of the time. I assume the packs you are talking about are the full daypack size, not just a "half-day" or whatever. When planning a big trip it can be hard to decided what to take and what not. I have been struggling with it for years. Every time I go day hunting for big game it's the same thing...but of course a bit different. That pack has to be prepared for much more than just having the right flies: knives, first aid, survival kit, rope, layers, water...
I fished once this past summer with my regular daypack on. I knew I'd be out all day walking the river and would need water, lunch, and somewhere to stash my jacket - late summer days range from 45* in the morning to 80* midday. In the end, it was pain in the ass. Having to take off a back pack to get to a different fly box sucks when you're standing knee deep trying to fool rising fish. Which is why I like the sling pack design and pulled the trigger on that Fishpond pack, it was too good of a price.
With that waterproof sling I can confidently carry my good camera, which I never do normally. I also am pretty serious about carrying water if I know I'll be out for more than an hour or two and I've never had a vest that was any good at carrying water and rain jacket. Those big pockets in the back don't let you get water when you want it without taking off the vest. Yeah, it's a minor inconvenience but an annoyance at any rate that I don't care for. The sling will swallow a water bottle and compressed light weight rain shell along with a couple boxes. And you can get inside of it while standing in the water. But it is not big enough to weigh it down like a pack.
