why no statements inside mixin teplates?

[via Digitalmars-d]

Is there a rational behind not allowing statements inside mixin 
templates? I know mixin does accept code containing statements, 
but using mixin is much uglier. so  I was wondering.

example use case:
//---------------------------------
int compute(string)
{
    return 1;
}
mixin template testBoilerPlate(alias arg, alias expected)
{
    {
        import std.format : format;
        auto got = compute(arg);
        assert(got == expected, "expected %s got 
%s".format(expected, got));
    }
}

unittest
{
    mixin testBoilerPlate("12345", 1);
    mixin testBoilerPlate("00" ~ "0", 2 - 1);
}
//--------------------------------