The biggest advantage of full-length daggers is that they are the lightest possible blade design for a given size. I carried an SOG Desert Dagger on a shoulder harness for a while, and immediately noted a huge superiority in confort over a similar size single-edge knife carried in the same way all day, despite the wasted weight of the SOG dagger's heavy pommel...
Now some single-edge knives are pretty light, but length for length, and blade stock thickness for blade stock thickness, the dagger will always be much lighter, which makes them particularly suited to concealed carry over long periods. This lightness is in my view by a wide margin the most important advantage they have.
Knowing this implies that a good dagger design should never waste weight with anything more than a stick tang and, preferrably these days, a full plastic handle devoid of metal guard or pommel.
Another essential design feature is that the compromise in cutting performance should be minimized with deep hollow grinds. Some Cutlery Shoppe exclusive Gerber Mark IIs were ground thinner at the edge and managed to still be sharp with a flat grind: They still wasted weight with the full metal handle...
The best dagger design I have seen (discontinued, but the one I am aware of that most closely follows the above criterias) is the Al MAr Shadow IV, though it needs to be fairly heavily sharpened when new. It is incredibly light for its size, actually feeling like it is made of plastic. The sharpest factory dagger out of the box is probably the Cold Steel Tai Pan, but it wastes weight with metal fixtures in the handle. Worst feature of the Tai Pan is the bulky oval guard, but I am still interested enough to eventually get one. The blade design is on the heavy side, but close to ideal.
Worst problem of double edged daggers is that traditional non-kydex sheaths will dull one edge, or the other, when drawing... This is immediately noticeable, if you always pick the same side to rub, after barely fifty to a hundred draws. They are already marginal cutting knives as is, so they are really the one type of knife that cannot afford any dulling... My solution, since I don't like Kydex, is that they are never drawn...
Some will say "the edge is not the point of these things". Well you can never predict what will matter, and a dull knife is always dangerous junk to me.
Gaston