I think allowing both extremes makes it clearer where precisely the center is located. Look at it this way -

In a balanced system you can see how far each side stretches. You're better able to understand what truly is the "extreme" and what isn't:

LLLLLLLLLL <you> RRRRRRRRRR

In a system where one side is censored, what you see doesn't change, but your perception of what you see is warped. The extreme L may not seem so extreme given how much closer you are to it. And even the mild R can begin to look more extreme compared to everything else you see.

LLLLLLL <you> LLLRRRR

Allowing both sides to speak without censorship may not have any impact on the actual L and R extremist, but it will hopefully have an impact on "normal" people and stop warping their perceptions.