Guessing Katana was chosen in combination with Power Crit and Superior Crit to increase the likelihood of a crit being confirmed and actually doing some increased damage (assassin daggers are x1, katana are x3).
Power Crit and Superior Crit won't increase the number of Mortal Strikes but when you're trying to MS something and it's making the save it's nice to at least be doing some decent damage to it.
Edit: And for the increased threat range with Stygian Razor as Khan pointed out.
Pwr and Sup Crit will likely be used to increase likelihood of MS, but it hasn't been designed yet.
I'm trying to see if I can do a Sai WM7 build, but would likely require Half-Kyton. Really it's hard to get decent damage on an assassin build, for a dexer rapier and katana are about the only choices.
you don't make an assassin for damage. You make an assassin for the insta kill. The minja was hurt in the last batch of changes and I would recommend a more classic route. (though the minja works well enough) I went with a rogue splash myself so I can be a picker as well as a trash killer, but that is personal preference.
Looking at this build and using it, I have to say I really like it. The hard part is finding the katanas to use with it. My question would be then, would switching this to kukri be a viable option or would the lose of damage be too great?