Thing is, whilst you are correct and that my given example should only compile to a single MOV instruction, you'd have to say that the original programmer would be at fault here (i.e. me) for conceiving such an appalling piece of code.  
I'd say that it would be my responsibility to optimise this particular piece of code.
It's similar to:
only a real banana head would code the above and so, the question is, should we really expect the compiler to look out for such redundancies?
One thing I love about PB is the speed of compilation. Try an optimising  c++ compiler - God it takes forever to compile anything!
It's a fine line I reckon.
I may look like a mule, but I'm not a complete ass.