Forgive me for being so naive, but what information does the vertical
alignment of an inline image provide?
I can't see how vertical alignment can assist the reader to understand
a document better - it merely facilitates an ease of readability just
as a formatted letter does.
Furthermore, I think your argument of vertical positioning can be applied
to text streams - indentation is a property of the text stream.
Not only that, but it is a property you will find active in any document
you care to read. Perhaps it's scope of use will persuade you to reconsider
its structural importance.