Now, I've always said that in most cases very few tools are necessarily needed - and a knife, useful as it is, is in most situations unnecessary. But when you actually backpack more than a couple of miles away from the nearest town or road, tools like knives become very useful indeed, and in some rare circumstances can be necessary and life saving. It's good to be able to do with minimal tools, but if you're going somewhere actually away from civilization, it is very unwise to not carry a real knife just because of its weight. There is a reason why a knife is one of the oldest tools made and used by humans.
In order to get the most out of a knife you have to know how to use it. Most people don't know what they can use a knife for. It wasn't until about 10 years ago when I got seriously interested in outdoor survival that I came to appreciate the value of a knife. You won't catch me out in the woods without a proper fixed blade now.
I also go fairly light weight - for a summer trip I'm less than 18 lbs w/ full water (48-64oz) and food (freeze dried) for a 3 day trip. I break it down like this:
Basics - shelter (sil tarp and bug screen), sleeping bag (rated for lowest temps +) ground pad, pack 3000c.i. (Old MS ghost)
Clothing - only what's required plus extra socks & jacket - all synthetic/ wool
Cooking - 1 TI pot, esbit stove
Essentials - first aid, PSK, rain gear, water purification, maps compass, FB knife, headlamp, TP
Food/Water - breakfast, lunch, dinner plus 1 extra dinner & plenty of water
Misc - camera
Basics & clothing change slightly w/ the season but cooking, essentials, food/water don't seem to vary much.
Adam
PS post a gear list if you really want some advice

PPS I also carry more survival gear than any of my hiking partners who carry 15+ lbs more than me. I'd like to see what the would do if they lost their pack and had to survive with what was in their pockets.....