Would I carry my knife that way? Not with the sheath I have right now, it's not built for that. That said, I do like having it accessible when backpacking so I lash it to my waist belt, point down.

I think the key is knowing your gear; there are sheaths out there that can be used inverted both safely and securely. With the proper gear, I think carrying your knife that way is a valid choice; you likely don't run the risk of loosing it or stabbing yourself any more than with any other carry method.

It's when you try to do things your gear isn't designed for that you start running into problems.