I think Wikipedia does a good job here.
http://en.wikipedia.org/wiki/Top-posting

Regards,
Xiaofan

http://en.wikipedia.org/wiki/Top-posting

Usage
Unsurprisingly, different online communities differ on whether or not
top posting is objectionable; but if it is found objectionable in a
particular community, top-posting in that community will generally be
seen as major breach of etiquette and will provoke particularly
vehement responses from self-named community regulars.

Objections to top-posting, as a general rule, seem to come from
persons who first went online in the earlier days of Usenet, and in
communities that date to Usenet's early days. Among the most vehement
communities are those in the Usenet comp.lang hierarchy, especially
comp.lang.c and comp.lang.c++. Etiquette is looser (as is almost
everything) in the alt hierarchy. Newer online participants,
especially those with limited experience of Usenet, tend as a general
rule, to be less sensitive to top-posting, and tend to reject any
argument against top-posting as irrelevant. A typical contrarian view
holds that their software top-posts and they like it.

It may be that users used to older, terminal-oriented software which
was unable to easily show references to posts being replied to,
learned to prefer the summary that not top-posting gives; it is also
likely that the general slower propagation times of the original
Usenet groups made that summary a useful reminder of older posts. As
news and mail readers have become more capable, and as propagation
times have grown shorter, newer users may find top-posting more
efficient.

Microsoft has had a significant influence on top-posting by the
ubiquity of its software; its email and newsreader software top-posts
by default, and in several cases makes it difficult not to top-post;
many users apparently have accepted Microsoft's default as a de facto
standard.

Perhaps because of Microsoft's influence, top-posting is more common
on mailing lists and in personal email. Top-posting is viewed as
seriously destructive to mailing-list digests, where multiple levels
of top-posting are difficult to skip. It is, moreover, nearly
irresistible to post an entire digest back to the mailing list, then
top-post a reply to that message.

Finally, top-posting is simply a custom, like wearing neckties or
eating with one's right hand, that serves to identify one's membership
in a particular community. This self-identification function probably
serves as much as any other factor to reinforce its use: one can not
expect much help in comp.lang.c++ if one self-identifies as a
"barbarian" by top-posting. In this way, not top-posting is similar to
other customs employed by other communities: the Unix community; the
various programmer "cultures"; the "New Jersey/Bell Labs", the
"MIT/Cambridge", or the "West Coast/Berkeley" "communities"; the AOL
"community".

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