I've carried a knife since I joined the Cub Scouts when I was 8. I did not get into knife training until I was in my 20. I had a little training in my teens, but not much, and mostly Karate disarms.
I decided to get training since I always had one with me as a tool. Even now I evaluate a knife to carry by how useful a tool it is. I leave my Gerber Guardian IIs, Fairbairn-Sikes, Applegate-Fairbairn, Smatchet, etc. at home in my collection. I carried an A-F Bootknife one summer EDC as an experiment.
IMO this thread is over-thinking the issue. People tend to like some things better than others. In martial arts, some people are really into kicking, some into wresting, others into hand strikes, others weapons, etc. I knew a guy who really liked all the fancy jumping and spinning kicks. What I do is very different from those I know who trained for years in the same dojo I did because we prefer different skills. Even though we all pretty much knew the same things, we each had things we stressed above others. When it comes to weapons, some people like working with sticks, other swords, some throwing weapons, etc. and some like knives. I knew one guy who really liked the Sai, but other than him it wasn't a popular weapon with schools I trained at or people I knew. I've not spent any time lately in a big dojo, but I remember when the Nunchaku was the most popular weapon by far, probably more popular to actually train in that all other MA weapons combined. Students who like knives probably just prefer them to other weapons, and there is no moral issue here any more than there is a moral issue with a student who prefers a Bo to a Nunchaku.