I also thinks it depends upon the activity one is doing wearing the sheath in question; as well as the size of the knife. if one is carrying a backpack it is a pita to find your knife in a hurry under the bottom of a pack. Plus the pack can put undue pressure upon the sheath 'loading' your belt with weight that transfers to a pain in your gut. now, if you were say fishing, or boating and just wanted to carry the knife back there for 'just in case' , not frequent use, then it can work for some. the danger is definitely in replacing the knife back into the sheath safely.
Otherwise, I'm a side carry type wearer of mid to large blades. A small enough blade worn in a horizontal sheath, in a 'cross draw style' as mentioned by another poster, has worked for me. The problem with larger knives above 3" blades is the sheath is larger out of necessity, and that in and of itself becomes a problem (for me) due to it's inherent bulk. I don't want to wear a large sheath in front of my belt buckle personally because if i'm out in the woods I have the large pack buckle there. it's a personal choice and there are some nice 'Scout carry setups' made by folks here, to each their own.