Yeah but that Cummins only turns 3,000 or so RPMs with not much of a cam either. They were originally designed that way because the injection pump timing was run off of the front of the motor and in a diesel the timing of the injection is critical to not destroying an engine. These days it's handled electronically and they could conceivably use a timing belt, but that probably wouldn't be conducive to a 300,000 mile major repair expectation.
But hey, it's your car, run what you want. No one ever said fabbed sheet metal valve covers were necessary either but they sell the hell out of 'em.