Didn't know the 4.2 engines had this. The first picture on this thread, very similar to the ones fitted to the 968 engines. Oil driven to allow the valve timings on the input cam to change, hence the rush you get above 4000rpm. With the Porsche, the chain can stretch over time and the chain guides wear. I seem to recall they are over a £1000 a shot and it they break (separate chain driven by the output cam), then you can add the cost of an engine rebuild too.
Whether it's a design flaw, I don't know, but it's my understanding that manufacturers like Porsche, Maserati, Ferrari and pushing the envelope and are usually the first to introduce innovations to drag as must performance and characteristics as possible.
We just pay the price when the cars get older.