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i think all the dialogue about the water pump got started when jake started seeing engine failures due to bits of deteriorated water pump impeller blocking coolant passages and killing engines.
as per porsche maintenance scheme, the water pump is one of the many items you replace upon failure and not on a scheduled basis (like brakes, engine mounts, shocks, tires, etc.). given that one failure mode of the water pump can kill the engine, jake recommended the 3 year replacement period (ie, replace it before it fails). 3 years may seem like over-cautiousness, but given all the variables that can affect the impeller lifetime (coolant type, heat/cool cycle, etc.) and consequences of failure, you have to assign some schedule to it.
there was talk of metal impellers to avoid the one catastrophic failure mode, but the only ones i can find are from a dubious vendor, and there is also talk of what damage can be done by the metal impeller to the engine if the bearings fail and the impeller starts chopping about ...
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