I understand your point, but I still think it is bad from the point of view of language paradigms[1]. Iframes should have not been created in the first place.. You are changing the purpose of the language while it was not made for it.
(yes in my view I interpret includes as a basic procedure)
Client-side includes are not "processing". HTML already has frames and iframes which do this, just in a worse way, so we'd be better off.