Knives have so many features and functions, as well as social connotations and there are many that are useful on the trail. It's much like choosing a pack or other gear and you want it to work for trip in mind, with cost, quality, weight and function all adding up to a choice. Do have some sort of serviceable cutting tool with you when leave the pavement; something like a single edge razor blade doesn't cut it for me ;)
What isn't needed is some massive Rambo chopper. You want a knife that can be used to prepare food, trim line or clean a trout. In an emergency, a knife can help to get to dry wood and make fuzz sticks and tinder to start a fire. You can use a small saw for starting cuts to snap off a branch or cut small branches for shelter-making. Batoning can be done with a fixed blade to split wood for fire making. I don't think much of batoning with smaller knives as a general practice. Bigger cutting jobs are better accomplished with a folding saw rather than a large knife, or a small ax or hatchet.
My personal leaning is a 3"-3/5" folding knife with a stout rust-resistant blade, a good locking system and a pocket clip. I carry a simple little Benchmade Mini Pika every day and I would feel just fine hiking with it. I do normally take a Benchmade Griptilian with a plain edge (non-serrated) blade.
Other knives I own and have used:
Moras. I currently have a Mora Robust (which I wrote up at http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=65919). I have used other Moras and prefer something with a slip resistant handle, which is where the more traditional wooden-handled Moras are weak, IMHO. I wish they made the Robust in stainless. The 4" blade is as much knife as I can imagine carrying. The Mora Craftline and Companion knives are similar and very useful. Moras are really inexpensive, simple and light for fixed blade knives. They will take an amazing amount of abuse. You can baton with one if you had to and the blade profile is great for food prep, repairs and bushcraft style woodworking. If you want a fixed blade, spend $15 on a Mora and don't look back.
Victorinox Little Vickie. This is really a paring knife with a razor sharp 3.5" serrated stainless blade and a simple slip sheath. They are tough enough that commercial fisherman use them for repairing nets, so they will do gear repairs with no issues. If you want a food prep and fishing knife, this is it. The design is far easier to keep clean and safe around food. They are inexpensive (~$10) and weigh one ounce total. There are a number of other paring knives with slip sheaths, that fit the same niche as the Little Vickie, like KAI, and Kuhn Rikon.
Opinel. I have owned a couple over the years. I have relegated them to a "picnic" knife for car trips and camping. My major falling out is having the wooden handle swell from moisture. and becoming near impossible to open. Many swear by them. They are like a folding Mora in some regards-- inexpensive, dead simple and perfectly useable cutting qualities. There have been some problems with the US distributor for Opinel and the factory lately, making them a little harder to find.
Swiss Army knives. I prefer the 111m frame models with locking blades and/or a saw. I'd love to see a 111mm frame Victorinox with a one-hand opening blade, a saw, scissors, and an awl. I carry a Victorinox Classic everyday and there's one on the "survival keychain" that I carry in my pants pocket when hiking. I like the Farmer and Hiker models for smaller non-locking versions. I have a Wenger Handyman that has the tools I really like: blade, saw, scissors and awl. The Victorinox Fieldmaster and Huntsman are about as multi-layer as I would want to carry on the trail.