Guard/No Guard- depends on the use of a knife. I have a combat tanto with nominal guard. Why? It was never, ever intended to stab anyone. Stabbing is not efficient use of that blade. Slashing is. Slashing can cut multiple people in one slash and keep going on to the next. I hear over and over and over about the armor piercing ability of tanto knives. Tantos never were intended to pierce armor. The curved, flat or convex ground tip meant that the tip would slash effectively and push aside the material as it was being cut. Nothing more.
A guard was used when blade on blade contact or the need to stab was included. If I were using my tanto as a backup to a sword or other blade I would have a guard on it to protect some my fingers and prevent slipping up the blade in a thrust (oh and if I were to stab with a guardless knife the only effective way is in the reverse ice pick).
As for other guardless knives, they can allow the ability to clear and choke up for close work. Guards get into the way. Kitchen knives have usually no guard....the blade is made in such a way that you don't need one, yet you can use the pinch grip if needed to choke up on it...incidentally that is the proper way to use a chef's knife when cutting stuff on your cutting surface.
I also will point out that no surgical blade in use that I know of has any guard of any kind. Imagine a scalpel with a bowie style guard...ridiculous right? I have chip carving knives and none of them have a guard for the same reason.
