Yeh, this is a sop to old style Unix mailboxes that got codified into the mail standard, basically.
Ancient history, if anyone cares:
The message delimiter was /^From /, so internal ones had to be escaped somehow. Prefixing "> " was how ancient /bin/mail did it, so that ended up in the standard to avoid disturbing such systems. It didn't change until Solaris 2 finally added a size to the header (in a rather annoying fashion).
Many mail processing tools, including some mailing list archive software, still assume this and will break if they're not escaped.