Why have pin nut?

The more common term for that type of nut is a "ring nut". They are typically used on larger diameter rotating shafts as having the corners of a hex nut whizzing around isn't the best, and they can be used at intermediate positions along a long shaft between other items (i.e., they can be tightened from the side with a hook wrench). Not a requirement here as it's on the end of the shaft, and, being recessed into the sprocket, a socket has to be used, but can't really fault the engineers for selecting it IMO as it's sort of industry standard. Others have identified a fine-pitch, small-cross-section hex nut that can be substituted, but that's still sort of a "special" odd thing that you aren't going to be able to get at the local hardware store..

 

It was SuperformanceUK who had the special hex nut substitution option for the cam drive sprockets:

http://www.superformance.co.uk/308/eg.html

Part Number 30815105 (down the page in the "Pulley" section)
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(but I'd rather buy the ring nut socket )

 

Actually I would ask the opposite question, why are ring nuts not in more general use?

They are good engineering. The force applied acts directly on a tangent of the nut and so the applied torque can be very precise. On a hex the force which is actually turning the nut is a result of other forces which are trying to compress the hex shape into a circle.

 

But the sum of the forces ends up the same, so the torque is the same ie, the wrench and the nut can be considered one unit, no matter what shape handles the coupling.