motorcycle theory...why a vertical twin?

I believe that the British vertical twin is really a design born out of the acceptance of the unreliability of the British single. I remember reading a short essay in a back issue of RIFLE magazine that compared a gun to an internal combustion engine. I was thinking about the British love of vertical twins and how it seemed to parallel their interest in double guns, both double barrel shotguns and double rifles. Unlike what many people now think, the double gun was invented not to provide two fast shots, but originally just to give one the reliability of two complete actions and barrel assemblies. That is, if one mechanism broke or otherwise jammed, one always had the spare barrel and lock. And in British motorcycle literature one can read endless stories of a twin limping home on one cylinder. It is design based on failure and an acceptance of failure as inevitable...and now we have the Triumph triples....  

British pre-unit twins had a reputation for poor reliability, too. I believe the vertical twin was added because it was smoother than a single cylinder motor.  
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1994 BMW K1100RS
2007 Harley Softail Custom

Less machine work on a parallel twin, V twin has two deck angles, two heads, two cylinders.
The brit parallel has cyls in one lump- heads in one lump.

Most all bikes from 1960 down were unreliable.