Also, it's a question of cost of bandwidth use - the more disk space that archives use, the chances are the more bandwidth will be used looking at them.
If someone can't find the answer in the archives and they then send a message to your list (which triggers how many on-list replies), how much bandwidth does *that* take?
IME, as long as your archives are *easy* to search, searching archives takes less bandwidth and gets the visitor the answer faster, which makes your list more effective for you and your visitor.
jc
