Why you cannot use an unsafe keyword in an iterator context?

Eric Lippert has an excellent blog post on this topic here: Iterator Blocks, Part Six: Why no unsafe code?

What I want to know is why you would use pointers for this at all. Why not simply say:

private IEnumerator<char> ReverseRead()
{
    int len = _str.Length;
    for(int i = 0; i < len; ++i)
        yield return _str[len - i - 1];
}

What's the compelling benefit of messing around with pointers?