On Indices (Indexes)

The Context
In my rant on pages numbers, I raised the issue of making a text easy for readers to use. I argued that page numbers are a nicety that helps readers use the text. I am now beginning to think that Indexes (Indices) do the same thing for readers, and I am noticing that not all texts have them.
My Beef
I tend to read lots of nonfiction texts that are really collections of essays. I am finding that more recently texts of this sort are not providing readers with an index. Personally, I think that readers of these kinds of texts would really find an index useful, but I imagine that editors of such texts don't always have the time or the money to do extensive indices.
Rant On
So, what do you think? Are indexes (indices) useful to readers? What can editors of collections of essays do to produce an index easily and cheaply?