OK, so I'll concede that maybe you don't really need something like this 5.7 oz., 5" SOG Seal Pup Elite:
But, what about something more modest, yet fully functional like my 2.6 oz. Spyderco Delica 4:
I use my knife a lot on the trail--more than enough to justify it's weight. I use it for opening food bags, food prep, cutting guylines, shaving branches to make dry tinder, digging cat holes, etc. It's undoubtedly the most versatile piece of gear I carry. Aside from sheer convenience, I just feel more secure having a real 3 7/8" blade in the unlikely event that I get lost and have to spend a few unanticipated nights out and go into "survival mode". I think it's also important to consider self defense from both animals and humans. A paper-thin razor blade isn't going to do much to fend off a mountain lion or hostile hiker. While rare, attacks do happen and in my mind it's better to have something that could be used as a weapon than being totally defenseless.
For those that say they've carried a knife for a long time and never used it so they ditched it, I ask if they would apply the same logic to a first-aid kit. I carried a first-aid kit for a long time (probably over a year) and never used it. The first time I actually had to, I was glad I had it with me--I would have been out of luck otherwise. I've carried one ever since. I'll bet most of us don't pull out our first-aid kits very often, yet still carry them becasue we realize that their weight is justified by necessity. I think the same holds true for knives. Even if you don't use a knife as much as I do for simple camp chores, etc., when you really need it, you'll be glad you have a real blade and not a miniscule razor.
In short, I feel it is unwise to head into the wild without a decent knife and the Delica 4 has served me well for little weight. Of course it's not going to protect me from every situation, but it's certaily better than having no knife at all. On a more practical level, I simply can't see sacrificing its convenience to save a mere two and a half ounces.
So what about the rest of you? Do you carry a knife? Why or why not? What's in your pocket?