Many feel that “interrupted thread mode” encourages greater care of thought, because it forces repliers to go through the quoted text and erase what’s not pertinent, and to reply to each point individually. Top posting is often done quickly and in a slapdash manner, sometimes leading to top-posters missing important points.
Top posting is Mistake #1 in the article "Common Mistakes in Email Messages and Usenet Postings and How to Avoid Them".
This Usenet style of posting has carried over to email, if you’re emailing people you don’t know. If you know people very well, eg. family, then you usually don’t quote at all – just write your email as if you were writting an old-fashioned letter.
For emails in corporate settings, however, the situation is different. Top posting is common; usually the quoted original remains totally unchanged. The benefit is that mails can be forwarded easily. The entire branch of the thread resulting in the current mail is contained as quoted material. This wastes bandwidth, of course, but is often a sort of audit-trail that shows who was involved, how the issue developed. Furthermore, as emails are not sent to the entire globe but to a selected list of recipients, bandwidth is not much an issue, these days.
On wiki, while “document mode” is the most-preferred style, but most conversations are in “turn based” style which is preferred over “interrupted thread mode”.
In Usenet, “document mode” is also the most-preferred style (taking the form of FAQs), but most conversations are in “interrupted thread mode” which is preferred over “turn based” (also called “bottom posting”).
Contributors: original author(?), piranha