If you were to purchase a Player's Handbook, how important is it to have an index or table of contents? From a design prospective they are extra print pages, but I know they are a help from a reader's perspective.
Very important for reference as you will need them to look stuff up. Not having them would be a major impediment. In practice, they are only a few pages long, so you really are shooting yourself in the foot if you omit them. If you really have to cram the book into a specific page count, drop the text size by half a point, the leading by 1 point, and adjust the tracking to -5 or -10. Then get ruthless about re-writing paragraphs with orphans, which will pull a bit of space out. Then re-write some of your more verbose content to be more concise, cutting down the word count. Pull out artwork if you need to.
On a shorter works of (say) 50 pages or less you might be able to omit the index, but anything bigger than that really needs indexing. Never omit the table of contents.