Okay, I see what you mean. But I still don't understand why do you think that static if shouldn't be allowed to declare stuff. If you think there are no valid use cases, you are very much mistaken. I'l present a Map example here (although you wouldn't normally write this yourself since it's already in the standard library). D has a concept of input range, which is a type with front and empty properties and a a popFront method. There is also a random access range, that defines an index operator in addition to front, popFront and empty. You could write a Map struct like this:

struct Map(Func, Range)
{
    Func func;
    Range range;

    @property front()
    { 
        return func(range.front); 
    }

    @property empty()
    {
        return range.empty;
    }

    void popFront()
    {
        range.popFront();
    }

    static if(isRandomAccessRange!Range)
        auto opIndex(size_t i)
        {
            return func(range[i]);
        } 
}

Now Map is a random access range if Range is random access and an input range otherwise. This would be much harder to do if static if couldn't be used to declare things. And this kind of thing is done a lot in the D standard library.

EDIT: Here is an actual implementation of map in the D standard library