But that's booooring.
Pit's moveset suffers so hard from just being the Palutena Bow with everything else potholed to the specials, and you don't get to see half of the things he actually uses.
Inkling's the same problem. Half of everything is just potholed into one move and the other half is just gone, and what do you get in trade? A dozen different really boring feeble attempts at attacking with the same weapon that don't have all that much different going on. And the Splattershot can do way less than the Palutena Bow ever could.
Even as far as the ink spreading mechanic goes. . .the Splattershot really has only one kind of shot. Ever. If they bothered with that mechanic, having other shot types would allow for other strategies in how to spread ink across the field. Splattershot isn't really that conducive to variety even without being locked onto a 2D plane. You've seen one shot of splatter, you've kind of seen them all.
It's not like I even need much imagination to see moves outside of the boxes that would suit non-Splattershot weapons better. A Dash Attack would better suit a Roller, a jab would better suit a Brush. Heck, jabbing is practically how you fight with a Brush! Do those things with a Splattershot and you've just got kinda. . .blah. The other weapons just outright do a better job than the Splattershot in those positions.