68,000 miles on OEM spark plugs

[rosemo...@yahoo.com]

During the summer of 2000, I bought a 1997 Corolla (1.6L, 5-speed) with
31,000 miles on it. Part of my inspection was to remove and inspect
the spark plugs to get an idea of the overall health of the engine. I
noticed that they are OEM plugs and looked beautiful. They were dry,
light tan, and showed no sign of erosion of the electrodes.

Since then, I have monitored the fuel economy as an indication of the
overall health of the engine. I consistently get 33 mpg mixed
city/highway, and 45.2 (yes, forty-five point two) mpg on extended
freeway trips. Every 10,000 miles, I remove and inspect the plugs.
Each time, the plugs look beautiful. I just shrug my shoulders, give
them a light cleaning with a wire brush, check the gap that never
changes, and put them back exactly where I found them. The car now has
68,000 miles on the OEM plugs, with no sign of erosion (last checked at
60,000 miles). I perform other routine maintenance, but so far, I find
no compelling reason to replace the spark plugs.

Given this history and the fact that the fuel economy continues to be
as stated, is there a reason for replacing seemingly perfectly good
spark plugs that have 68,000 miles on them? Yes, I know plugs are
cheap, and I can afford to replace them. Why should I?

P.S. This is a great car. I have owned it for nearly 5 years and, not
including gas and oil, I doubt I have spent more than $100 on routine
maintenance. Most of the expense in maintenance is filters and such.

Thanks,
Brian