If you include *any* kind of protection, you change the "game" hugely. People with head protection will move and fight differently than those without. I imagine Dog Brother style stickfighting would be a lot different without the headgear. WEKAF eskrima players would change the close range they usually fight at with no head and body protectors (and heavier sticks...)
Sparring, no matter how realistic, is a training method, not an all-out fight. This is why any really traditional sword schools in Japan (for example Tenshin Shoden Katori Shito Ryu, the oldest school in Japan) do not spar. The only way to be true to their art and training, would be to use live swords. And that is a very quick way to run out of training partners.
Sparring is a tool. It develops timing, control, stamina and a host of other attributes. The level of protection you use is a compromise between safety and the intensity you want to achive. The groups I train with use headgear and hockey, or street-hocky gloves and padded sticks (smak stiks). On the whole, our system is blade based, and as such, the hand is a primary target. Wearing hand protection allows us to strike a small, moving target with as much power as we can. It still hurts sometimes. If you want to add some "reality", or in fact merely appreciation of pain, you can use lighter gloves and heavier/harder sticks. There can be some benifit in the fact that if you spar like that, you *will* try and move your hand more.You have to balance this with the potential for disabling injury.
In short, make your training methods fit your style, but be true to your art without endagering yourself, or being unrealistic.
(Rant over
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