In a skill challenge, however, a PC with poor skills is an actual liability. The party's chances of success are lessened by that character's involvement. In fact, you don't even have to have poor skills to be a liability--you just have to have less skill than the most skilled PC in the group! The logical thing to do is to have all less-skilled PCs absent themselves if possible. The equivalent in combat would be if the fighter alone were more likely to win than the fighter, cleric, wizard, and rogue together.
First, just in case there's any doubt, I can absolutely see what you are saying here.
But second,
from the point of view of skill challenge implementation, I think this is absolutely getting it backwards. That is, if the skill challenge is framed and resolved so that the most logical thing is to have the fighter (or rogue, or bard, or whatever) tackle it solo then it has been poorly designed.
What I mean by that is that - and to continue the comparison to combat - is that
if all the monsters lined up one-by-one, then having the fighter take them on would be more logical than letting the wizard take a poke. But no one sets up combats like that (but for the occasional exception like the arena fight). The ingame situation is set up so that all the PCs are engaged, and if the fighter alone is surrounded then s/he will go down.
The skill challenge needs to be set up the same way - so that, in fact, the PCs are better off having all the PCs do stuff because otherwise the party (and the players) will fail in their goal. And part of the play of the challenge - and this is why I don't entirely agree with [MENTION=6688858]Libramarian[/MENTION] in saying there is no game aspect - is shifting the fictional situation so that (for instance) the fighter or barbarian has something to offer even in a predominantly social challenge.
Instead of a "before X failures", a hit point like system? Even if you fail, you don't lower the total?
That wouldn't fundamentally change things - in Burning Wheel, for instance, social confict is resolved via a "Duel of Wits" with a hit point system, but having weaker PCs on your side makes it more likely that you will lose hp, and hence lose the challenge.
A hp system does open up the idea of healing, but skill challenges already have that, in the form of secondary checks to remove failures.