Weapon Immunity from the old days was designed to make creatures that were invulnerable to certain levels of characters, even if they were on equal footing otherwise. They were the Referee’s “don’t go this way or you’ll die” wild card when all else failed to keep the party from going “that way”. Most parties of players had the common sense to retreat or come up with a non-combative plan to deal with such creatures. Sometimes though, through unlucky treasure rolls, the players just aren't equipped with some of the proper equipment. In my sandbox, I don't like to present the players with roadblocks. If they decide to go where certain death awaits, they should always have a chance of success (not guaranteed, but if a situation where a decent plan and hot dice will save their butts and they are having fun, then let the dice fly). Some people don't like Energy Draining, others detest Petrification...my pet peeve was always Weapon Immunity.
One thing about it that didn't make sense was that it was independant of the creature's power level indicated by Hit Dice. Hit Dice were used to measure everything else about a creature: hit points, 'to hit' numbers, saving throws, spell ability (in most cases), etc. Weapon Immune creatures were immune though, whether they were 2 Hit Dice or 20, large or small, if you didn't have the right weapon tough luck for you.
The 3.x edition changed those rules (or in some circles it was called 'killing that sacred cow') so that monsters were merely Damage Resistant but could still be harmed by the weapon they were not resistant to the damage of. Unfortunately, those numbers were skewed on the high side in 3.0 and then lowered in 3.5, but they still didn't function with a creature's power level.
When working on Onn, I like the concept of resisting some damage but didn't like the arbitrary concept of the newer rules (they used a block of HD to determine the amount of DR, closer to what I was looking for, but not quite it). Eventually I settled on using the actual HD for the DR number. It made sense - if you had 2 creatures, one with 2 HD and one with 5 HD and both are resistant to damage from non-silver weapons (even if it's magical non-silver) then why should they both just be immune? Hitting both creatures with non-silver weapons should still have the ability to inflict some damage - the 2 HD creature would be taking 2 points less damage where the 5 HD creature takes 5 points less damage, but if you're capable of more damage than the reduction value, then a few points is a few points. If the character doesn't inflict more damage the the reduction value, then no damage is inflicted on that strike.
It may not be as simple as telling the player none of his blows are having any effect whether it was 3 points or 13 points, but it's more fun for the player who has been hunted down and cornered by that werewolf and doesn't have a silver weapon. At least he knows he has a chance, however slim.