On Voltaire and free speech

One of the aphorisms often attributed to Voltaire, albeit apocryphally, has him assert "I may disagree with what you say, but I shall defend, to the death, your right to say it".

This is generally offered as evidence or how principled and high-minded he was: prepared to sacrifice his life for the rights of others; but I'm not convinced. Rousseau, maybe. He was loopy enough to say something like that and mean it literally... not to mention feckless enough to chicken out if it ever actually came down to the choice in question.

But Voltaire was a consummate pragmatist. If he had turned the phrase in question, I am pretty sure it would have had a second half:

"I may disagree with what you say, but I shall defend, to the death, your right to say it... because it's the best clue we'll get as to whether or not you're a dangerous fool."