The question is fairly simple. Universally, the Bayonets present on Splinter Rifles take the form of, presumably monomolecular, blades. However, triangular bayonets have been shown to be the most effective for rifles, so much so that they are banned from modern warfare.
A harder to stich wound, much wider to cause far more blood loss, and presumably, larger amounts of agony from the victim.
Given that the Kabalites already have knifes as well, it can't be to serve a dual purpose.
Perhaps the reduced level of bloodloss is advantageous? Better chances for the target to live, more cargo to return to the Eternal city? Perhaps the shape of the blade is irrelevant, it's the toxins on it that do the real damage? Outlandish as it maybe seem, could it be that in the eyes of the model designer, a knife was more aesthetically pleasing?