I also agree personal letters would be more effective.
Depends. A lot of the time, sheer quantity is much more important than individual quality. Some people don't have the time to write their own. Also, most of the time the letters are being read by a secretary/assistant who summarizes the general gist for the legislator. In that case, it doesn't matter if there are 20 different letters or 20 identical ones.
And I agree with jimbob: what'll it cost? 10 minutes and 44 cents?