An agnostic objection to atheism

Michael Lurski, of Bethlehem, Pennsylvania, has a letter to the Editor of Newsweek this week in which he adduces a problem with atheism:

Atheist author Sam Harris makes a strong case when he criticizes established religions for the harm they have caused and for the dangers they now present ("Beliefwatch: The Atheist," PERISCOPE, Oct. 30). And he is correct that belief in God is irrational and cannot be proved by reason or current evidence. However, it is just as irrational for Harris to advocate atheism, a certainty that no God exists. An atheist can no more prove there is no God than a believer can prove God's existence. The only alternative based on reason is agnosticism, the belief that it is impossible to know whether there is a God without sufficient evidence. It does not deny the possibility; it is uncertain.

Harris, however, addresses this objection in The End of Faith, but I no longer have a copy of the book to consult. I seem to recall that it went sort of like this: There are any number of things that might exist, whose non-existence we cannot prove: purple people-eaters, and the like. Shall we say that we're agnostic as to their existence as well?

That isn't exactly an ironclad argument in favor of atheism.