This is all kind of underscoring the whole My Precious Encounters thing. The encounter, as designed, is too important to sacrifice to changes the PCs bring in the environment.
You say "important" as if that's a bad thing. Let's replace "important" with "giving the players the exciting encounters that they crave" - then how is it bad GMing to do that?
Why bother really doing anything smart or unexpected to change the situation if no good comes of it?
Because you, as a player, may not be playing with an eye to changing the odds of future encounters. You may be playing with any eye to doing interesting stuff in this encounter, and changing the story of your PC and of the campaign world more generally. Your PC is not going to earn the moniker "the guard ganker" unless s/he ganks a few guards, and that story element is completely independent of how the GM balances or rebalances any subsquent encounter, on the fly or otherwise.
Your reply, for me at least, reinforces my earlier reply to Rogue Agent - this concern about rebalancing encounters on the fly in order to maintain mechanical pace and excitement makes sense from the point of view of Gygaxian, operational play. But it is just wrong to project that one preference for play over D&D or RPG play as a whole, and imply that anyone who is running a game where the pleasure and the fun come from something else is making some sort of error.
Why would the players have any idea that taking out the two guards didnt make the fight easier, even tho you made two guys appear out of nowhere?
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the reward is 2 extra guards of xp.
That's one way to look at it. I'd be happy to tell my players, if they asked, that I tweaked the encounter on the fly. (They might guess, too). In my own game I tend to see XP more as a campaign pacing thing than a reward, but if you play with XP as reward then getting two easy kills would certainly be a benefit of ganking the guards, even if the subsequent encounter is then rebalanced.
Because it's fun, and for the kudos of showing off to your friends how clever you can be?
This. Also, because you might think it matters whether or not your PC is a guard ganker. (This seems to have different implications for a paladin or a warpriest, compared to a rogue or assassin, for example.)
I do not accept you contention about the "razor edge" nature of 4e encounters. I have never experienced it. Also I dispute that an encounter must be tough enough for a TPK to be interesting or exciting. This is however, just my experience and your views and experienced could be very different to mine.
My experiences are the same as yours.