And suddenly one fine day, selective quoting was invented, and everything could be read in its proper context. ;-)
Honestly, top-posting (especially combined with quoting entire previous posts, relevant or not) is such a _bad_ practice. It makes following the discussion needlessly difficult.
Time and again, when I find someone committing such sins, I at once assume they have nothing to say that's worth reading, and skip to the next post. So I guess it's self-regulating. :-)
Ask yourself -- which do I prefer: a concise post, or yards of blah?