There is a bit of confusion here.
\medmuskip around binary operation symbols; whether the space around the two + signs in
A+(2n+1)B should be the same or different is a stylistic question. In my opinion the spaces should be the same.
\thinmuskip in various places, most importantly at the sides of an operator under certain circumstances. An example is worth several words in order to show it should never be set to zero:
Is this enough of a reason?
\medmuskip, here is a comparison:
Note that TeX won't use the spacings related to
\thickmuskip in subscripts or superscripts, but will still insert those related to