This depends very much on your backpacking style and, with that, what sort of other gear you are carrying.
I've done an awful lot of backpacking (New England, CA) with just a Victorinox Classic. Yes, just the tiny key ring knife. The scissors are super handy for bandages (blisters) and the little blade is fine for cutting open food packages and parachute cord. These days, I carry a tiny Leatherman which adds small pliers to help with stove repair (not that my Svea ever needs it). I've done a lot of miles augmenting that with a tiny Camillus serpentine jack.
Now, I always carry a locking folder in my shorts/pants pocket. Mostly I use it for food prep like spreading peanut butter, cutting bagels or cheese and the like. But also, I'm just happier having a knife with me. My choice is the Opinel #9. A #8 would do or so would a larger #10. But the #9 fits my hand well and doesn't frustrate me when digging peanut butter out of the jar like smaller knives do. The Opinel is very much up to the task of making feather sticks and even splitting thumb sized branches should the chance to make a fire present itself. Here's a shot from last weekend, ski touring with the family up on NH. The Opinel #9 handled the cutting and splitting for the small wood for that fire just fine.
Campfire by
Pinnah, on Flickr
If you're going to be relying on making fires and relying on cleaning fish... well... then you open another can of worms, so to speak. If you're going to be on the water or dealing with ropes, you may want to consider a non-traditional rescue knife.
IMO, a small micro tool or folding scissors should be in every 1st aid/repair kit. That along with nail clippers (to deal with toe nails).
Beyond , what you carry should fit your style of hiking and should make you happy. So, if you want to eat freeze dried food and lug around a big ol' fixed blade to slash open the package because you like the big knife, do that. Seriously. The only wrong move here is getting hurt or hurting somebody.
If this thread gets moved to the Wilderness and Survival forum, a) you'll get good advice there but b) note that the view points there tend towards the bushcraft/survival crowd. Nothing wrong with that and lots of good stuff to learn from them. But you may want to balance the input by hitting hiking forums, especially those that deal with Colorado. Find out what people are carrying on the trail there.
I think you're right about SAKs. Their locking Trekkers make a lot of sense (but I'm sticking with my Opinel).
edc-pairing by
Pinnah, on Flickr