In pair programming, the experience of every member of the team can be spread to new member. This experience is always in sync with the code, because the "senior" of the pair knows how the code works and what the design is.
So what is the utility of design documentation in this case ?
UPDATE
I don't imply no design, I imply no documentation. With a team which practice pair programming I think that everybody is disposable, because everybody knows the code. If the senior developer leaves, I think that there is always at least one person who knows the code, because the experience was shared before.