Only my own opinion, but this a more of a matter of educating admins. We know that every admin should have their own unpriviledged account, and only switch to root mode for administrative tasks requiring it.
In addition, the smarter the mail reader, the more complex and consequently the higher risk of security flaws => no mail in root's box will prevent an admin to use the last beta but so cute version of [insert your prefered reader here] with root priviledge.
For example in a default FreeBSD installation, many crontab jobs send their output into root's mailbox. It is simply expected that the admin will not add clever rules there and will just use the good old standard BSD
mail program to read it.
TL/DR: my opinion here is that it is just an additional defense line to prevent usage of a security broken mail reader as root - not even speaking of automatic mail processing as root...