@wavelength - basically, I'm thinking about the idea of armor protecting from all sources of damage, not just physical damage (even if i make it more effective vs. physical damage). Why do you view this as such a 'weird fringe benefit?" Most equipment in my game is 'ordinary' so, without extremely rare items, how would a knight equip himself to defend against the elements if heavy mail and sheilds don't even give some protection against them?
Well, because...
To be clear, my phrasing of "magic resistance" doesn't mean to specifically exclude MDF....whether it's through MDF, or through elemental resistance, it's more of a question of should armor protect vs. elemental damage, or strictly physical damage.
...this wasn't clear in your first post.

When you say "fire resistance", "frost resistance", etc., I infer this means that certain elements will be doing less damage based on the equipment you're wearing (and these are fringe benefits because they will each be useful in a very small percentage of situations); this is an entirely and radically different question than "should armor provide only DEF or also MDF".
So if the question is actually "should a platemail also provide MDF", it belies a more fundamental question, which is "
what is the purpose of differentiating physical versus magical damage in your game?"
Most games don't seem to have a real good reason for doing this, and I'll admit (with regret) that I can't give a great answer to the question in my own games, even though I do make that differentiation. The closest I can get is in my most recent game,
timeblazer, where you have several choices to make on what to equip your characters with (and yeah, some armor in this game provides much more MDF while other armor provides much more DEF... and others still provide little of both but have conditional bonuses), so I like being able to give the choice of covering a character's (somewhat arbitrary) weakness or improving their strength. Still, if I were to redesign the battle system from whole cloth, I'd probably get rid of this distinction entirely.
A much better case for having physical and magical damage can be found in some MOBAs, like
League of Legends. Here, you buy your armor over the course of a single game (which can be thought of like an extended 45-minute round of combat), so if one player on the other team is growing really strong, or the other team overspecializes in mages or gunmen, you can give yourself a chance by specifically itemizing armor that can reduce the type of damage you're taking the most of.
Anyhow, unless you want to remove the differentitation between physical and magical damage, you should go with whatever helps make them unique, while remaining simple and intuitive for the player. If the player will be able to make a lot of choices between "do I use this slot for the platemail that provides more DEF or the breastplate that provides more MDF", then yeah, go for it and let "banal" armor increase your MDF. If the player's choice will be closer to "I only have one choice for each armor in this tier, but I only have enough money to upgrade the 'body' mail that increases DEF or the 'accessory' bracelet that increases MDF", then the game would probably be better served by not adding MDF to the standard defensive armor (because it would make most of your equipment types too similar).
Like a lot of mechanics questions, it can be distilled down to "will this enhance - or dilute - the core 'engagement' elements of my game?".