For one thing, most GUI mail readers will automatically convert the `>` into nested block quotes. For another, it's not common to see more than 2 levels of `>`

For one thing, most GUI mail readers will automatically convert the `>` into nested block quotes. For another, it's not common to see more than 2 levels of `>` (your text, the thing you're replying to, and the thing they're replying to); and yes, when it gets past 4 it begins to get really unreadable, so my gut feel is that it's extremely rare (although I wouldn't be certain without doing some sort of data mining). As I said, the main advantage of using email is the flexibility you have in editing the response; part of learning how to use the review workflow effectively is learning how to present the context; which means trimming the replies to balance information and readability.