I think you missed MY point KD or you just keep ignoring it. The whole thing about secondary skills is just an example of the concept that the SC system doesn't anticipate one character making all the rolls with the other characters simply aiding them. It doesn't make sense if you interpret it that way. REGARDLESS of what the effect of using a secondary skill is why would ANY character ever do anything except aid the guy with the highest skill bonus on the easiest skill in the SC?
Sorry, I thought the answer to this was obvious.
If I have a huge bonus with a skill, it makes sense for me to often try to use it as a secondary skill if I can think of a way to do so. Many PCs have one or two skills that they are generally better than the rest of their skills and sometimes better than the difference in difficulty between a primary and a second skill check.
If none of my skills are primary for a given encounter, then I should also be participating and that means two options: Group Skill cooperating for another PC, or trying to figure out a secondary skill.
If I am really good at History and the DM allows History as a secondary skill check, then it often makes sense for me to try to make the skill check because I have a huge bonus. I could fail, or I could succeed. But, I am the only PC with that huge bonus to History, so it sometimes makes more sense for me to use History (my best skill) even with the fact that secondary skills have a higher DC then it does to Group Skill someone else.
Anyone in the group (and then limited to one or two PCs) can Group Skill any other check during that round. So while I am doing my secondary skill check, so other PC should be Group Skill cooperating for other PCs.
In fact, it makes a lot of sense for other PCs to Group Skill my History check since it does have a higher DC.
But, the entire skill challenge will be easier if each "round" of the skill challenge focuses on just a few skills with some PCs duing skill checks and other PCs doing Group Skill than it does if every single PC tries to do a primary or secondary skill check every single "round" (note: I quoted the word "round" since each skill can be done over an extended period).
This is no different than combat. Smart DMs have minions sometimes Aid Another and flank with the big nasty instead of attacking themselves.
Smart players Group Skill on some rounds and do skill checks for their better skills on other rounds.
Why would someone be stupid and try to do a skill check every single round, without at least trying to get occasional help from your allies?
In my home game, players are cooperative. They don't care who kills the foes, as long as they are contributing to the group effort. Ditto for a skill challenge. They don't care who gets the success, as long as the group succeeds.
And note: a Group Skill check is a skill check, so that player is having his PC do a skill check every "round".
Whether it’s the use of a primary or secondary skill, or whether a character is cooperating to help another character make a check, every character participates in a skill challenge.
The rules obviously allow and encourage both. Each round some PCs should do primary or secondary checks and other PCs should do cooperation in order to maximize success.