Wow! I just left this little question here and what a response! I really appreciate all the thoughts posted.
Fairly enough, I should say that in my case currently this pertains to the Scion conversion I posted about previously as that's the game I'm running right now that needed help using shields properly, BUT I'm going to be running another game out of DFRPG soon and I wanted to make sure I knew how to work shields for that game because I have a player I just know is going to want to use them.
Crusher_bob seems to have pretty much laid it out simply as possible.
From what I know of historical warfare, the shield stopped being carried on the battlefield with the advent of plate harness (i.e. full plate armor). Instead of a single handed weapon and a shield, people instead started to go for two handed weapons that could damage someone in full harness.
So, a rules space to look at would be:
Someone in plate harness (armor: 2) and wielding a pole axe (weapon: 3) has an advantage over
Someone in plate harness (armor: 2) with a sword (weapon: 2) and a shield (does whatever).
But someone in mail armor (armor: 1) would prefer a sword and shield over a 2 handed weapon.
As grainy as the rules are, lets see if they allow anything to fit into this space...
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Option 1:
The shield provides +1 armor.
That gives the 2h, plate guy weapon 3, armor 2; and the S&S, plate guy Weapon 2, armor 3.
This makes them even vs each other, they both have a net weapon vs armor advantage of 0. So, not so bad
Now, how about the mail armored guys?
2h, mail guy has armor 1, weapon 3 and S&S guy has armor 2, weapon 2. So the trade offs remain the same.
This produces a simple rules space where the trade offs are simple, but doesn't produce any 'bad' results.
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The only problem with this is not really in the way it works, but in the way it works in the circumstance in which I was using it. Essentially, the character using a shield was Tora, the daughter of Thor. She's already ... really tough... and obviously trained in weaponry. So making her even harder to do damage to... well it wasn't really working out as a +1 to armor so we sought out other options. (She has armor 2 from Supernatural Toughness and wears 1 armor rating clothing that were made for her by a friend.) Giving her a total of 4 armor really seemed too powerful since she was already hard to hit in the first place and now almost no damage was being dealt and even really powerful enemies couldn't land any damage off her track to a consequence. She faced some really nasty opponents and was all on her stress track as she killed them.
Option 2
Shield adds +1 to the defensive skill
This option makes the shield a much more attractive option because the shield is effectively 1 armor, in addition, it reduces the number of hits you take, so it's always better the trading up to weapon: 3. Not a great choice.
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When we did it this way, it made her even harder to hit and reduced the damage because of less overflow in the attack. My mate really felt this way was too powerful and so did I so we just stopped using the shield at that point, setting it aside until we knew what to do with it (and I made this thread). (Lesser foes REALLY had a hard time with her, whereas otherwise they could at least knick at her.)
This could really just be an issue so much because the character in question is already so, so tough, BUT... This is exactly the reason we need to know what is balanced/fair and what is RAW.
What stops a Supernatural Toughness character from wearing thick armor and a shield and being utterly impossible to hurt?
There are no penalties for wearing armor, no penalties for wielding a shield...
I understand the in-game reasons. Assume that the in-game reasons have already been made reasonable in the setting and think about the question as such.
In this setting, yes, it is weird for someone to walk around with a shield, but much as Michael carries his sword around in a duffel bag, there ARE ways to disguise what one is carrying. PLUS there's a big difference between trying to walk into an airport with a sword and board and whipping out a shield from a pick-up truck as one is entering a hidden compound that's full of Giants and Titanspawn. As long as she gives it a look around and is sure that there aren't any cops etc nearby when she gets out, she won't get questioned again for the scene in all likelihood. And Tora? Being the kind of person she is? If she got stopped by a cop and asked why she's wielding a shield and a big hammer, she'd honestly just tell them she was defending her friends and probably make a social attack on them (Which, her Presence ain't shoddy!) to talk them into leaving her be. She's done it before successfully. If she ever got to a point of needing to be "taken in" because a cop saw her with her weapons and thought she needed to be looked into, she'd honestly probably end up knocking them out and getting away from the scene.
I dunno. I'm just saying that she gets around the weirdness. Our setting is a bit different so that it makes sense. Yes, Tora has ended up on the news and yes she's had to deal with the consequences of becoming temporarily very wanted in an area because she killed someone (that the mortals can't possibly know was actually a monster trying to eat them) and yes, she and her friends have had to deal with some serious consequences as a result of her really just not hiding her armor and shield and weapons at certain times.
But at this point I think I'm going to end up rolling with...
that the shield is just a part of her overall "armor". Her armor isn't full plate to begin with. It's usually something like a kevlar shirt or a small plate "sports bra" type breastplate made of molded, reactive armor. The point being that her style isn't full-cover armor. So the shield could really just be flavor, a part of her 1 armor rating from the protective gear she wears. And I'd be happy enough with that. Maybe if she went full defense, she could get an extra armor or defense out of it.
Her shield, for the record, is a mechanical shield that extends and detracts to go from a buckler/small shield size to a medium shield size. (Think of the awesome shields in Gears of War 2. Similar in concept if not in style.)
And yes, shields can be broken. >:3
I definitely wouldn't make shield use a stunt though. Just to me, that seems kinda like too much cost for something that just anyone could pick up and it'd at least help.
Also for the historical viewpoint: You DO NOT straight-up BLOCK attacks with your shield. If it's a blow from any heavy weapon (axe, hammer, etc) you will BREAK your arm that way. Shields are used to parry/deflect blows aside to keep your arm from taking the shock.
Those speaking from the point of view of having faced shield-bearing opponents in a larp, it's kinda a bit different when the weapons are all made of foam and latex. People CAN and DO hide behind a shield and just let blows fall on it an d be untouchable (my mate has a shield and uses it sometimes and maaaaaan does it suck to fight the shield) but in all realism (for the sake of re-creating the effect in a game when the shield isn't blocking boffers) you can not ever assume that just hiding behind a shield is going to save the wielder trauma. It might even get them hurt. They have to parry the blows with it. That's why armor is a pretty appropriate representation.
For example, an Amazon was being attacked by a thrall with a shotgun. She used a shield to block some of the blast and took damage past the shield (armor 1, reduced the damage but she still took some stress). Very appropriate. The shield just causing the attack to miss her (like if it added to defense rating) would not represent that as well to me.
Okay... I think I know how I'm going to handle this for my game currently. I'm glad it could open such a nice discussion though. Thank you to everyone! I really appreciate all the input.