Here’s a September 2007 online exchange with The Times’ editorial page editor Andrew Rosenthal, who is asked directly why Margaret McGirr gets so much play in the paper. From the response:

[W]e do publish her letters from time to time. The reasons are simple — she writes pithy and often witty letters, with a strong conservative point of view. That’s something we always look for as a balance to the liberal letter writers who predominate on the page, because they predominate among our readers. She helps provide a diversity of ideas. Who wants to hear only the point of view he or she agrees with? By [t]he way, we generally do not publish the same letter writer more than once every two months.

I get that they want a diversity of ideas, but when one end of the argument comes repeatedly from the same person, how much diversity can you claim?