I'm still not convinced of their utility as a stabbing sword. Since it is a curved sword, a large portion of the force applied in stabbing with one wouldn't be directed into the blow.
I will point out some people mistakenly call the Tachis a Katana.
Tachis have a much more noticeable curve, making them fairly dificult to stab with.
Stab/skewering attacks are used, can't cut through Armor, just stab him in an opening right?
Although I am not sure why people only think that Legionaries stabbed. In fact, slashing attacks were also a part of the Roman repetoire. I wouldn't actually say that the length of the sword is really what matters. I don't know enough about the difference in length between the average Katana and say, a standard spatha used by later troops. The spatha didn't really change Roman tactics that much, so I don't know what a katana would do.
I don't know much about Roman swordsplay....
Spatha were between 75 cm to 1 meter.
Katanas were 60-73 Cm.
so it's a 2-27 Cm difference in blades.