Against. Simply because...
Fighter: "I'm trained in war, but I was never trained to use armor."
If you don't want to use it, then don't. If you do, go ahead. It's like saying I never play females, so wouldn't it be nice to have sex choice as a trait?
/female disparity thread reference.
If it fits for your game I would do it. My group would not accept the trade though since the AC would be needed. Well maybe if they were an archer, but not if they were in melee.
Fighters are already the most feat-rich class in the game. I see no need to bloat their feat bag even further.
Use a fighter archetype that isn't proficient with heavy armor?
Feat starvation at low levels is already the most difficult part of the game to manage for most interesting concepts. Letting a fighter who would only wear light armor anyway gain three or four feats for doing so allows that fighter to become a combat god at early levels.
I suppose at high enough levels that feat bloat starts to occur the extra feats aren't that big of a deal, but I rarely play at that level anyway.
If my GM allowed this it would be hard to justify playing anything but a fighter at level one to grab all the feats you can (up to what, five?) and then pick whatever path you want from that point on.
I allow players to drop medium/heavy armor proficiency, armor training, armor mastery, and bravery to get best reflex and evasion at 7th level. Most of the players in my group really lean more toward to mobile/skirmish types than tradition knight in full plate. There really should be an archetype that supports a nimble character that doesn't trade class features for full attack, and doesn't take away weapon training. All in all, I think too many archetypes take away weapon training which is the only thing that makes the dang class playable to begin with.
The point is, a single level dip in fighter becomes pretty much required for any non full spellcaster. Getting 3-4 feats just for a level dip is well worth it.
And if an option becomes required, its not balanced.
Instead of a feat, you could allow fighters to choose between:
A.) Medium and Heavy Armor Proficiency.
B.) Canny Defense: When wearing light or no armor, a fighter adds 1 point of Intelligence bonus (if any) per fighter class level to his Dexterity bonus to modify Armor Class while wielding a melee weapon. If a fighter is caught flat-footed or otherwise denied his Dexterity bonus, he also loses this bonus.
I think it would work well enough.
I have an observation: Every class has a package of abilities that could be broken down into feats. There are many classes with redundant basic abilities that could be cashed in for extra feats. But the abilities were meant to be taken as a set. That is how the game was balanced. If you want to part out a class into feats then you should ban multi classed characters to prevent abuse. Otherwise what is stopping feat farming by switching between a couple combat classes? You could gain all that armor use back with a single level dip...
