According to API609, the butterfly valve shaft to disc connection shall be designed to prevent loosening due to vibration.  What will happen if loosening occur?
There is a dynamic torque on the vane of a butterfly valve whenever it is in other than the full-open or the full-closed position. This torque acts to close the valve. It is most pronounced on flat-plate (rubber seated) valves.  ALso, swirls and other turbulence in the line rattles the vane continuously.  If the vane comes unsecured from the shaft, it may possibly slam shut, with water hammer forces great enough to shear the stem and take the vane to do something really expensive downstream. 
Even if that does not happen, The valve will not perform its intended function if it is not firmly secured to the shaft.
There are many ways of securing the vane to the stem.  Tapered pins driven through the centerline are the most obvious, but lowest strength method. Torque plugs, tangential pins, and Gibb Keys all present greater surface area to resist shear without weakening the stem as much as drilling thru for tapered pins.  

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