why are Assign Expressions r-values?

[BCS]

Why is this not allowed

foo(inout int a);

int a;

foo(a=1);

The reason the compiler objects seems to be that the "semantic value" of 
an assign expression is the value that was assigned. However if the "semantic 
value" were the LHS (the variable assigned to) then in most cases the effect 
would be exactly the same, however the expression could be used as an l-value. 
Mostly this would be useful with functions that take inout args but some 
other uses might exist.