Yeah long post - if you don't feel like reading it at least check out the first three and the last video links. Those are modern attempts at recreating the fighting styles the medieval soldier would have used when trying to kill his enemy - extrapolated from a combination of practice, 14th century manuscripts for the training of soldiers, and a little bit of physics. It can be surprising how they differ from what we often think of - some of which is just hollywood and some of which is because a lot of the styles of armored or sword and light shield fighting we are familiar with were meant for tournaments where it was not usually ideal to kill your opponent but where looking impressive was. The real use of light and medium shields is quite a lot like Solus and other poster described them earlier. Tower shields though are probably far less useful than nearly every RPG rules system would have you beleive.
It would be cool yes... In other threads if have stated my oppinion about how i think the shield should work. Suming up i think blocking should effect stamina and when this is drained you should be unable to block. My point is, as you say, if i catch a blow by chance the drain on block should be bigger then an intended block. but its a good idea
You might findhttp://www.youtube.com/watch?v=7-Ms9RMH7IA&feature=related. He's one of the instructors and researchers at Hammaborg (part of HEMAC) and has done most of their work in recreating and researching fighting techniques for the viking round shield in particular. While HEMAC (Historic European Martial Arts Coalition) tends to take themselves only slightly more seriously than SCA (Society for Creative Anachronism) who accounts for most medival comabt demonstrations people are likely to have seen - Hammaborg seems to put a lot more effort into researching the original fightbook manuscripts as well as exploring the biomechanics of wielding and fighting with these implements.
Anyways these 3 videos constitute an English language demo they did specifically on the viking round shield:
http://youtu.be/7-Ms9RMH7IA
http://youtu.be/RzTsGYFzMLY
http://youtu.be/JPpYDEBvaiQ
They're pretty interesting compared to the bulk of the SCA and other re-enactment/enthusiast demonstrations out there because he does a lot of explaining of the mechanics and the reasoning behind the design of what he's using and the techniques they require or allow. You might particularly like the part where he explains that shields do most of the job of protecting the users simply by the virtue of being there at the ready. "Blocking" movements are slight and subtle adjustments when the shield is the proper size for the user - that is it basically makes attacking the side it is wielded on pointless for your opponent. So yeah, blocking is much less active than a lot of people tend to assume - for the most part that is accomplished by virtue of the shield being there and held properly. The main reason to do any significant maneuvering this particular size/weight shield about is because the Viking round shield fills an anologous roll to the main gauche dagger would for a duelist with rapier - protection first but also an offhand weapon.
Some of their other videos touch on the idea that a shield that is good for a lone fighter or single combat is generally not a very good shield for use in regimental formations. The maneuverability of the round shield that allows it to be used as such an effective off hand weapon is neutralized in those situations while it also lacks the thickness to protect again a decent bow. Against an opponent in heavier armor and wielding something like a sword and heater sheild in single combat it would be quite effective. Just pray they don't have a hammer or a mace instead of a sword. These videos here give you an idea of how much less maneuverable and versatile the heavier shields can be - that is they are often best left in position doing what shields do rather than being swung and swiveled about.
http://youtu.be/-rrvCFbJseA
Even though this isn't quite as informative as the other videos - it does demonstrate exactly what you describe in terms of how blocking with a decently hefty metal shield works. It also actually illustrates a good logical reason for why you would take damage through the shield - heavy shields like that may often need to be held in such a way that the shoulder of the wielder is used to support the weight - and so some of the energy imparted on the shield is transferred in an uncomfortably direct way to the meat-bag behind hit it and their poor collar bone.
But yeah, in the case of the heavier metal shields wielded by hobbiestists or the light Viking round shield wielded by someone who seems to have researched the topic extensively - the readied position that the shield is held at for a good deal of the fight is roughly the place where it needs to be to offer protection. As you suggested holding it where it needs to be is a workout. After watching the 3 videos above (the SCA one is less informative I kind of hope the designers have watched them. I could see the shield and a perk or two being useful for avoiding the frustration of an NPC who seems just too adept at blocking your attacks.
Oh and take a look at this. It's funny to see how effective a hand-and-a-half or bastard sword can be when you wield it like you'd expect someone to wield a staff. It does make the ridiculously wide crossbar on those swords seem far less ridiculous too.
http://www.youtube.com/user/DierkHagedorn#grid/user/2D4363CB9ECD68CD
But hey - these videos should also help show why realism should not be the goal either (not saying you were arguing it should be). Waddling around in platemail would be tedius, metal shields would be suprisingly boring and limitted, tower shields might get you killed when out fighting on your own, and arrows would go right through the lighter more maneuverable shields anyways.