Katanas since it's inception has always come in two categories, the "kunai" knife-type, and the normal, longer katana variant. The former usually is a smaller, lower delay, lower damage (but similar dps weapon) that would normally provide some stat boost. The other was for main hand dps purposes. You can see that trend was started with artifact weapons (you get two of these katanas, one of each type).
When they went ilvl, I guess they wanted to keep the trend and have the kunai types with a lower skill/delay. It hardly matters though, since they give such a high amount of acc that usually makes up for it.
With that said, Shigi is really good for lower enmity/high acc/daken, and just a great all-purpose offhand katana. It gives you flexibility with DT and the unique Ninjutsu boons, which turns it into one of the best offhands to have. Of course, as Langly mentioned, you want to obtain all of the ones he mentioned. I happen to have 3 other Katanas that I use for situational use in addition to the ones he posted, but the list he identified is a great list of off-hands to reach for.