i don't want to spend too much time discussing technical/UI issues on LBO, so i will restrict myself to one response:
its not just an issue of bandwidth. its also one of readability. your point about commercial email requiring reference is unclear to me. there is nothing about top posting that cannot be replicated by a more considerate posting style (such as the interspersed dialogue style).
there are definitely valid applications of top posting (a fwd'ed message is one). however, most actual uses of it (top posting) rarely have such justification. OTOH, IMHO, for email discussion threads, where writers and readers are many-to-many mapped, a dialogue style is more readable.
> I googled "top-posting," not knowing for sure what it was, and according
> to one discussion, it is not top-posting itself that offends but failure
> to clip (which often attends top-posting)
there is also the lack of proper quoting/attribution. in that area, the level of inventiveness i see on pen-l and lbo is quite impressive, and unmatched by everything i have seen before on the net! ;-) there's the one where the quoted text is left unmarked but the response is prefixed with the author's name. there's the variant where "response:" is used instead of the name. there's another where "=============="s are used to block of sections of quoted text. then there are the messages where the quoting is done quite nicely (with ">"s) but there is no attribution (no mention of who is being quoted). its a pandora's box.... ;-) ;-)
--ravi