I find I agree with Spook on game mechanics more than I feel comfortable with.
Seriously though, stamina is a terrible idea. Ya, it's realistic, but I don't get on my computer and play video games because I want total realism.
The effects that stamina would have:
1. Less spam: Good, maybe? Though as I say all the time, spam is VERY EASY to counter unless you just screw up, or they have better footwork. For crying out loud, I have a 69 speed weapon and people rarely outspam me. The only time you can get outspammed while performing better footwork than your opponent is an extreme case like GMaul vs. katana, but that's the big downside of a weapon like the GMaul.
2. Less techniques (feinting, spinning, etc.) Ya, they look kind of silly when done over and over, but without these, fights between good players take forever. That leads to #3:
3. People avoiding tough fights, and chasing easier kills even more than they do already. Good player vs. Good player fights take time, and a fair bit of technique usage. This means that if they choose to fight each other, they won't be as capable later. No one wants that.
4. Less footwork. Terrible TERRIBLE consequence here. Footwork is one of the most important differentiating factors between a good player, and a bad player, imo. Why the hell would you want to reduce this factor?
TL:DR: Stamina does nothing but reduce the skill in the game. "Oh, but you need to conserve stamina" is a bullshit excuse. All the "conserving stamina" means, is more boring fights, and people avoiding good players more.
Besides, these aren't multi-hour long battles, save in strat (where you respawn). A trained warrior isn't going to get tired in 5 minutes. Hell, even some scrawny, malnourished peasant won't get tired that fast.