I, too, am in favor of putting reasonable limits on the frequency of authors writing to the editor.
Both Chris' and Max's letters have already expressed much of my opinion on the issue, so I won't have to say too much.
In order to have a fair and balanced opinion section in the paper, you need to have more than just a few of the same authors writing in over and over and over. I would like to see more fresh new ideas from a wide variety of authors on a regular basis. If not, I suggest you cut back on the amount of times a week you have the "Letters to the Editor" section.
I'm sure that you could find something else to fill in there the rest of the week. Just like Chris, I look for the author at the bottom of each letter. If it's from a frequent writer, I skip over it.