Although I have no training or real time experience with the double-edge dagger, I would like to add few points to the other opinions:
1) Originated from the Middle Ages in Eu, it served as a self-defense and second blade after the STRAIGHT sword. In certain areas of the world (even in Eastern Eu where sabre ruled) it never became popular. As far as I know, it was originally designed to finish off the injured foe by piercing between the armor plates into the vital organ at the end of the fight (quite cruel). Where heavy armor was not used (Ex. Middle East), it never became popular.
2) Because of that it was designed for piercing SOFT MATERIALS. That is served by the pointy tip that is rather weak for anything else (piercing armor-tanto rules, praying in a tactical or survival situ).
3) LACKS any BELLY for cutting or skinning.
4) LONG SLIM BLADE is good for reaching any organ even through thick clothes but lateral (?) weakness is a downside for survival.
5) Tang is even thinner and therefore weaker.
6) DOUBLE HILT is a plus in self-defense (to catch a blade?) and to avoid slipping hand onto edge on either side but makes difficult positioning your thumb for pushing the blade or for delicate whittling.
7) DOUBLE EDGE is a drawback when splitting wood. But a big plus in knife fight when both pushing and pulling the blade can inflict severe injury.
8) ROUNDED/OVAL grip might be fine for a thrust but not for a sure grip in a wet palm.
9) SKULL CRUSHER pommel -as it says- might be fine for getting bone marrow out
but not for hammering on it.
These are some of my learnings from here-and-there. If the use of this type of blade interests you, I would turn to the Tactical or Self-defense forums for additional info.
BTW, as I had chance yesterday to read through the SAS Encyclopedia, I saw the old issue survival machete on a photo mentioned among the essential gear. Well, nothing fancy, rather like a beaten up wooden handle sugar cane machete. If I remember correctly seeing the new isuue model in another SAS survival book, it looks more like a bolo with a forward curve with Kraton-looking handle.
Sorry, if it came out too long.
HM