Good tip! I learned that from the pre-purchase inspector who inspected our house. He said they're optimized to prevent rust just long enough to get through the warranty period, then bam! Water heater's toast. So by replacing the anode rod frequently, you increase the life of your water heater indefinitely.

Oy, my house is 118 years old! I don't even WANT to know what some of the piping looks like. Fortunately it had all be replaced at some point, at least it's all copper.