the main reason why aluminum RR's wear faster is because( i think someone already mentioned this) of the hardened steel against a softer metal(aluminum). since it is not depending on an oil wedge, there is more friction. a good example would be an overhead cam motor. notice their isnt any cam bearings on an OHC aluminum head. aluminum is a decent material for a camshaft as long as it holds a true line bore and good oil wedge. many cars have this and have hundreds of thousands of miles on them with little or no wear. i will use my 87 Toyota truck as an example with 182k on it. now put needle bearings in place of the oil wedge and see what happens.
also let me give you another example. i was doing an intake job on a 97-98 Chevy Venture van with a 3.1(maybe a 3.4). from the factory it has a roller cam and roller rockers which were Steel, not aluminum. this van had 125k on it. no signs of play at all with the rockers.
of course other factors do contribute to wear of aluminum RR's, like heat, maintenence, and to much spring pressure to name a few.
but what do i know........
