Even if we removed stunts from combat, every PC would have a
fairly decent list of actions to perform no matter what the circumstances
of the particular fight happen to be. On a skill challenge based on social
and knowledge related skills, the jock types help the party most by sitting
out of the encounter and letting the brains handle everything. This would be like having those skilled at diplomacy sitting out of combat because they would be a liability to the party and couldn't help.
Maybe I'm different, but I've never designed a skill challenge that didn't have some way for everyone to help.
If my players are trying to convince the town mayor of something, sure Diplomacy and knowlege skills might be primary, but I always put in something else that will help from the physical skills....
Some examples:
Endurance, taking some political big wigs out for drinks and getting them drunk to free up the conversation (but still remembering them all)
Streetwise or Stealth to find out what pressure points the mayor may have
Athletics to get in with the guards and have their captain put in a good word with the mayor for you.
Acrobatics to entertain the people and take their mind off x problem to give the mayor some breathing room.
All of these things could add bonuses to the final diplomacy check.
No I don't use all of them on a given skill challenge, but with a little bit of thought you normally can build the skill challenge so that something falls into each character's wheel house.
PLUS: the other thing I've found that works well, is to tell the players they have to do x, and lay out a situation. Then let the players tell you what skills they want to use and how they will use it. If it's a good idea let them try and give a +2 to the next major check.
An example in one of our games, we had to entertain one of the fey courts that in return for some information we wanted.
We had a paladin, a wizard, a warlock, warlord and Ranger.
We started out the skill challenge by mingling with the court.
Knowledge Arcana (Wizard), Diplomacy (Paladin) and Insight (Warlock) were all used to give some successes and determine what skills would really be useful for entertaining the court.
We figured out that the main skills that would be used was diplomacy to tell a story and Acrobatics for tumbling. Athletics could be used for feats of strength but it was harder to impress the fey with this so we decided not to go this way.
We used Knowledge History (Warlord), Nature (Ranger) and Arcana (Wizard) again to give a bonus to our Diplomacy (Paladin) check on the story.
Then we used the endurance (Warlord) and acrobatics (Ranger) to put on a tumbling display.
In the end, every character contributed and we all had a blast figuring out how our skills could be used in what could have been just all taking/Diplomacy.