They climed that a Cessna is build to crz at a partucular RPM (75%, or whatever RPM the owner stipulates), and increasing power above the planned crz setting puts undue stress on the engine, thus higher maintenance costs, increased risk of failure etc...
Others may disagree, but as long as the engine is operated inside the limits set in the POH I would say that is a load of codswallop. Often you operate at an RPM lower than max, but this is mainly due to the way the fuel consumption goes up, also of course, if you operate
at your max RPM it only needs a slight descent or updraft before you exceed it so a good idea to have a bit of a buffer.
