In reply to Chris Aldrich.

Another benefit that I always forget is some of the stress of starting and setting up a new notebook and planning on how many pages to leave for the index and future index portions. Index cards are generally much more flexible and don't require much (if any) thinking ahead. I also don't have to remember which notebook/index I may need to search in to find anything since it's all in one place rather than potentially spread out over several notebooks.

When I do need to look back to look up things in old notebooks, I usually jot down a reference to that older material by notebook number/page number into my new index in my index card box for future easier searchability.