I support this suggestion!
If you want to balance this out so the game doesn't become too easy, just lower the "main" defenses by as much as you increase the other ones.
Alternatively, at least make some mails that have a balanced defense in everything, but maybe lower total (the ones vendors sell is a good candidate).
Armor used to give you an assortment of all four defense types. The game has changed a lot since then, but if that had not I'd guarantee it would be far too easy.