They are also I think, just something one should have because you never know if u r gonna need it... I recall mountain biking and walking the bike down a real steep hill I wasn't comfortable riding, and slipped on a rock. Hurt my arm. No way out with my bike, cradling it with the other. I cut a sling from some cord and then I was able to walk my bike outa there.
Or once at work, a box on an overhead shelf caught on another. The other was full of a 2 ton (rated,not weight lol) car jack. Both had started to fall, and I could barely hold em up. I needed to cut the strap that caught on the other box before something huge fell on my head! A boxcutter was useless and so would have been a nick knife. But used my spyderco with my one free hand and cut the strap and avoided a car jack falling on my noggin.
You never know if you are going to need a knife, but if you need it and don't have it, you could be screwed...

I also think the mechanical aspect has something to it. A lot of the guys I know who are into bikes (and actually work on them, not just some bloke with a lot of money and cervelo he can't change a flat on) are also into watches, or guns, or vintage stereos...or knives. I think there is a certain type of person that is fascinated by mechanics, blade profile, lock mechanisms or even just the history of the thing.
Lonerider