Obviously, if you clean and replace grease at regular intervals (say, one year), you may not notice much/any difference, but there is no data to support the idea that all greases are equally good because "contamination and washout is the real issue" as suggested above. Im sure contamination and water is the main offenders, but in not at all convinced all greases perform equally well on the long haul and especially in the presence of water and dirt.

Similarly motor oil are subjected to many different contaminations that degrade the oil, how ever some are much less affected and degraded than other and modern oil "last" many more miles than older formulations.