One thing that top-posting is good for
Like many Internet people of a certain generation, I don't like top-posting (and often have a semi-visceral reaction to it). For a long time I avoided it almost entirely but these days I'm in an environment where I'm reasonably exposed to it, and I've come to realize something about it.
Ultimately, the purpose of all quoting in email is to provide context, to remind people of the previous discussion if they don't remember it off the top of their head. Top-posting optimizes for the case when you don't actually need this context while still letting you recover if you do.
If you don't need the context, you don't want to waste time skipping over quoted text; top-posting puts the new text right at the top, so you can read it immediately. Plus, if you only need a little bit of context, the most recent context is at the top of the quoted text, right there for you to read next. (If you need a lot of context, top-posting is horrible.)
Realizing this caused top-posting to make a lot more sense to me, because at least around here, the common case is that people don't need the context very often (but it's unpredictable when they do want it). Thus, top-posting is optimizing for the common case.
(No wonder that it is so persistent and pervasive.)
This is of course yet another example of the fact that people are rational (well, generally). When people do crazy things, it's quite possible that they're actually completely logical and sensible from the right viewpoint. You just have to find it (which I think is often harder than it looks).