[Euclidean]

Although I never could understand why the heck someone would want a blade that is half sharp.

Greater structural strength, superior edge geometry, ease of use (try caping with a double edged knife), etc.

Double edged knives will always be more fragile, harder to use for many tasks, harder to sharpen, etc.

I avoid them personally to avoid any possible conflict with Texas laws, but even if not for that I doubt I'd carry one. A design with a sharpened false edge (the second edge doesn't go all the way down the blade) seems like a good compromise if you need a pig sticker.

 

[Euclidean]

But even at that RSSZ some of the greatest fighting blades in history have been single edged, such as the Bowie knife.

To be sure the double edged knife has its inherent advantages too, but it's not a clear cut (pun intended) case of "This one is better than the other!". Personal proficiency counts for a lot more than the design of the knife. It should also be pointed out certain martial arts styles tend to lend itself to a double edged blade.

It is worth remarking that if a knife is solely dedicated to personal protection, then a lot of bets are off. A lot of things don't matter that otherwise would. It depends on the person and their needs if a particular design is any good or not.

I'm just playing the devil's advocate. There's a use for everything.