Andrew -- the shape of the knife, the handle material, the intended use, and user preference all play a part in the sheath design. For example, a double gaurd knife can be put in a foldover pouch, but not as good a fit as in a double seam. A very tacky handle like the Rats can be fitted to a pouch, but can't be wet-formed as well as a harder handle. NO slight intended to the standard Bandicoot pouch sheath, but you can get a very tight fit which holds the knife fine, but can't be drawn easily. Or vice versa, a loose fit which doesn't secure the knife well, but which CAN be drawn easily. I love pouch sheaths, but they don't work well on the Res-C grips (my opinion)-- but on the little Busse it worked great.
For a long time I avoided keeper straps and snaps whenever possible, just something else to get cut by the blade. But the Rat series sheaths solves that problem, allows a secure fit, and a quick easy draw.
Bottom line -- the sheath has to work as part of the whole rig. I try to choose a style that works with each individual knife. On wet's sheaths the design has some modifications for style, but the basic ideas are the same.
Mike
just to keep them in rotation! 
