> Another common use-case where (as with the AMS \\ in maths)
 > something non-standard seems intuitive is things 
 > '\item-like \foo s' 
 > Where
 > \foo [arg]   has the standard 'argument meaning'
 > but
 > \foo
 > [Paranthetical text starts here. ...

right, this is what one really would want with such type of commands, but we
all well know, do going within the limits of the TeX parser (and for xparse I
think the TeX parser is a given).

My question was more is there any other command within the 2e world where
people have applied the amsmath redefinition for looking for an optional
argument (ie without gobbling spaces).