There is an obvious, and always overlooked, fundamental advantage to double-edged fixed blade daggers: For a given length, they are, especially if combined with a stick tang (any self-respecting dagger should always feature a stick tang), by far the lightest possible knife blade design. Some models however will ruin this huge carry confort advantage with full tangs, or a heavy metal handle like the Gerber Mk II for example.
This dagger weight advantage is extremely noticeable in some concealed carry options, especially with shoulder harnesses: No other blade design removes so much weight from the blade stock, and the advantage is so noticeable after prolonged carry that it is to my mind the premier reason to choose this blade design for self defense.
Sharpness can be marginally acceptable both ways if the blade is hollow ground, but it is usually below acceptable when flat ground. Some of the "Cutlery Shoppe" exclusive Gerber Mk IIs (with green handles) were ground thinner at the edge than the standard models, and these were acceptably sharp even though flat ground on a narrow symmetrical blade. Their green handles have a very rough sandpaper-like finish that indentifies these better made models... (Cosmetically, the centerline grind was rarely perfectly straight unfortunately, like on most/all Gerber MK IIs)
The best dagger design I have seen is the Al MAr Shadow IV, which comes dull but can take a surprisingly effective edge because it is hollow ground. The all-plastic handle capitalizes on the inherent weight advantage of daggers: This roughly 13"+ knife feels unbelievably light, almost like the weight of a Bic plastic pen... It is longer than a Randall Model 14 at a -tiny- fraction of the Model 14's weight, yet shares a similar initial blade stock thickness...
The SOG Desert Dagger is a close second to the Al Mar, but wastes weight with a very heavy pommel.
The Cold Steel Tai-Pan is probably the best and sharpest "useable" dagger currently available. At 10.5 ounces it is unusually heavy however, but worst of all the extremely bulky oval cross guard precludes serious concealed carry consideration (in my opinion). It is probably the only production dagger that can genuinely double as a using knife, but its handle has poor grip retention due to the constantly tapering shape.
The one less than obvious, but very serious, downside of daggers, never mentionned yet extremely significant, is as follows: On a single edge knife you can always widthdraw the knife from the sheath by forcefully pulling the knife to one side, rubbing the spine on exit from the sheath: This eliminates damage to the edge when taking out the knife. On a double-edge dagger (unless the sheath has "pinched" edges, like a Kydex sheath, where neither of the dagger's edges can reach the sides to dull themselves), you will, without extreme care and slowness on withdrawal, inevitably dull one edge or the other on withdrawal. The trouble is, these knives are so marginally sharp to begin with they cannot afford much dulling in the first place... The dulling process is very obvious within less than a hundred draws, if you choose to always rub the same edge...
Kydex usually solves this problem, but personally I don't like Kydex, since it often scratches blade centerlines, is noisy, hard to the touch (and thus unpleasant to wear next to the body), and it often just looks awful to my eyes.
My solution: Sharpen them once, carry them rarely (if ever), and never pull them out of the sheath... I use and fondle my single edge knives instead... Shoulder harnesses make the most sense for large daggers, since they don't get in the way, allow for longer blade lengths, and are a weight-sensitive form of carry. Large daggers are very nice to own, are uniquely suitable to prolonged carry, as some kind of lightweight "back-up", but other than defense I don't see why they should be used...
Gaston
