Is there a reason for that? I was just thinking of having the default number of sides be a highly composite number. Something like 24, 36 or 48.
I don’t think there’s a particular reason for that. It might just be yet-another-developer thing where 32 is a power 2 (2⁵).
It takes a second to change it after adding it. What if you need a cylinder with 32 sides? Or a 5-sided one? What is default doesn’t matter in this case.
Having 32 sides makes it reasonably smooth after using smooth shaded without having to add a subsurface modifier. That’s my guess anyway.
Make shortcut to add cylinder and set vertice count as you wish.
That would be my guess, it seems like a good starting point, which I would assume is the goal of any default setting 
I always thought that it is because 32-sides is pretty smooth without too many polygons.
I have no idea, but I would prefer 36…
And…why?
Or, make the cylinder eight (8) faces in the T panel when you add it. Then use several subsurfs and pull up the edge split modifier after making smooth. Uncheck ‘Edge Angle’ but leave ‘Sharp Edges’ checked. Then pull up the edge menu after selecting an end loop and select ‘Mark Sharp’. Then the other end. You might have to switch the modifiers in the stack.
i’d prefer 42…
( as 42 is the Answer to the Ultimate Question of Life. )
LOL, guys. It doesn’t matter why someone wants the number of vertices they want. They want what they want and no amount of discussion is going to change that. Personally, I’d like to change it on a project-by-project basic because it’s just one more tedious thing that has to be done if I require more or less than 32 for a given object.
The types of responses in this thread remind me of what Henry Ford said about automobile color: They can have any color they want as long as it’s black. 