In the light of the Patrick Mercer affair, this is worth revisiting. 

Our merely social intolerance [of dissentient opinion], kills no one, roots out no opinions, but induces men to disguise them, or to abstain from any active effort for their diffusion. With us, heretical opinions do not perceptibly gain or even lose, ground in each decade or generation; they never blaze out far and wide, but continue to smoulder in the narrow circles of thinking and studious persons among whom they originate, without ever lighting up the general affairs of mankind with either a true or a deceptive light. And thus is kept up a state of things very satisfactory to some minds, because, without the unpleasant process of fining or imprisoning anybody, it maintains all prevailing opinions outwardly undisturbed, while it does not absolutely interdict the exercise of reason by dissentients afflicted with the malady of thought. A convenient plan for having peace in the intellectual world, and keeping all things going on therein very much as they do already. But the price paid for this sort of intellectual pacification, is the sacrifice of the entire moral courage of the human mind. A state of things in which a large portion of the most active and inquiring intellects find it advisable to keep the genuine principles and grounds of their convictions within their own breasts, and attempt, in what they address to the public, to fit as much as they can of their own conclusions to premises which they have internally renounced, cannot send forth the open, fearless characters, and logical, consistent intellects who once adorned the thinking world. The sort of men who can be looked for under it, are either mere conformers to commonplace, or time-servers for truth whose arguments on all great subjects are meant for their hearers, and are not those which have convinced themselves. Those who avoid this alternative, do so by narrowing their thoughts and interests to things which can be spoken of without venturing within the region of principles, that is, to small practical matters, which would come right of themselves, if but the minds of mankind were strengthened and enlarged, and which will never be made effectually right until then; while that which would strengthen and enlarge men's minds, free and daring speculation on the highest subjects, is abandoned.

John Stuart Mill On Liberty