I don't know that I would say that tip down carry is bad. I prefer tip up carry, but I carried tip down for years because the knife I preferred only came in that configuration.
The main reason I like tip up carry is because it puts the knife hinge down inside the pocket instead of sticking out of it. I found that the hinge tends to stay much cleaner with tip up (hinge down) carry, especially if you're working in a dusty/dirty environment. I really hate the feeling of having a knife grit/grate when I open/close it, so maybe that's more of a motivator for me than for some.
I also think that tip up carry puts the knife in a better position on the draw. I have two knives that are essentially identical, one is configured for tip up carry and the other one can only be carried tip down. That lets me experiment. With tip down carry, I have to reposition the knife significantly in my hand after getting it out of my pocket. With tip up, if I put my thumb into my pocket down the side of the knife, the knife comes out of my pocket very close to already being in the right position for use. I don't see that as a big deal, but, at least for me, there's a noticeable difference and the difference favors tip up.
I agree with the comments already made about tip up carry and snagging. You definitely want a knife with the kick adjusted properly so the tip is below flush with the handle and a closure design that has a decent bias towards closure. Without that, a knife carried tip up could snag the tip on the pocket during the draw and is more likely to open in the pocket than if it were carried tip down.
Some of the nicer quality knives you buy these days give you the option to pick tip up or tip down carry by changing the position of the clip. About a quarter of my pocket clip knives offer that feature. They're all configured for tip up, for whatever that's worth.