Almost a pointless thing to do.

I consider mower spark plugs to be good for the service life of the engine, but what I worry about is them seizing in the head, so I like to screw them out occasionally. What's silly about my attitude is it doesn't matter if they become stuck in the head if I never intend to change it.

 I did the same with Daisy's plugs the other day, they now look quite used but good. I really am concerned about long life plugs like these being in place for 100k km, so I'll pop them out from time to time.