Let's run down the Fighter's trained skills to see how he can help out.
Athletics: The Fighter climbs to a vantage point out of sight of the patrolling guards, and after the Rogue makes a Stealth or Bluff check to draw their attention, he drops down behind them and dispatches them quickly.
Why would the guards have a blind spot where the Fighter can do this? How did he get there in the first place without being spotted?
In a drawn map combat encounter, the chances are that the DM wouldn't do this because it doesn't make sense from a defensive standpoint. Are the leaders of the enemy group really that stupid to allow this?
Endurance: Same situation as for Athletics, but instead of climbing up, he's holding his breath in the swamp (a la Schwarzenegger in Predator).
Again, why would the guards have a weakness like this that leads to the guard post?
Heal: The Fighter concocts something from his healing kit that the Rogue can sneak forward and slip into the guard's water supply, making them sleepy or otherwise easier to dispatch.
Would you allow this in a combat encounter? What happens if a Fighter doesn't have a healing kit? This too is a stretch.
Intimidate: This is the only tricky one... Maybe the Fighter could devise a plan that will shock the guards into silence just long for his friends to circle them.
Again, would you allow this in a combat encounter? From 100 feet away where the Fighter was before being spotted? Or is the Fighter right in front of the NPCs in order to "shock them"? The skill isn't a Fear spell.
Streetwise: The Fighter uses his knowledge of how guards usually patrol to find a blind spot in the schedule, which allows them to advance to a more advantageous position.
Streetwise is actually a skill that takes an hour to perform. Again, not something that could occur quickly or as the skill is written.
I'm getting the feeling that some people are opposed to the use of group Stealth checks in general... if one person fails the roll, then the group is spotted, right? Is there no room for mistakes?
The purpose of group skill checks is to assist if assistance is possible.
The idea of the Fighter assisting the Rogue in Stealth when the Fighter himself is lousy at it seems far fetched.
But in a skill challenge, group skill checks are precisely what players will attempt. They will also attempt to rationalize the most ludicrous of ideas when doing so.
Your examples here reinforce, at least in my mind, why skill challenges for combat are not a good idea. The ideas you wrote above:
a) are a bit far fetched. You had to really stretch to come up with them. If you as DM had to do so, so will the players.
b) introduce terrain and other conditions that may or may not have been part of the original scenario. For example, your Endurance idea for the Fighter require that the swamp end up close to the guard post, even if the original map that you designed did not have this. In this regard, the skill challenge becomes a self fulfilling prophecy. Because the skill as described by the player requires that a barrel be in the alleyway, a barrel is suddenly mystically in the alleyway (different example here to illustrate the point).
c) were thought out ahead of time. The players have to come up with these ideas on the fly. That's not always easy to do.
One of the problems with skill challenges to begin with is the "bang, you're dead", "no, bang, you're dead syndrome". It's playing cowboys and indians with the dice deciding whether or not someone's idea not only wins, but also is possible of existing. If the skill roll is a success, then the Fighter manages to get in close, even though the skill isn't stealth. If the skill roll is a success, then the Wizard can fake out the opposition with an Arcana roll to create an illusion (your earlier example), even though he normally could not do this at all and doesn't even have a single illusion spell. If the rolls are successful, then the enemy leaders are suddenly idiots would have no tactical training whatsoever, regardless of how the DM set them up in the first place.
As you can tell, I dislike skill challenges to begin with (e.g. Hammerfast has a skill challenge to walk to the other side of a set of normal woods, arrgghhh). I think they are overused and they allow some players to shine while other players sit around picking their noses or being forced to try weak skills as group skill checks. In a combat encounter, every player gets to shine in his/her area of expertise.