Back in first edition you needed silver or better weapons to damage them. No amount of hitting with a mundane weapon would work. But since everyone past level 3 or so had a +1 weapon, the werewolf's immunity was trivial.
I do think that making it impossible to kill werewolves without silver is a bad idea (similarly, I think it should be ok to kill trolls without fire or acid...just difficult). Remember that in folklore sometimes all it took was a small amount of silver and *thud*, dead werewolf, like it was super-poison-kryptonite to them. This would be a situation in which it is impossible to kill them unless you had some silver in which case they would be a trivial threat.