I read something recently (I'm not sure where) about all the people who are out there writing books now. I know when I look at Barnes and Noble or Borders, I get overwhelmed simply by all the titles that line shelves, and I realize that is only a small fraction of the books that are published each year.
Should fewer books be written? Certainly there are more books than the market can support. Yet, the act of writing still matters to the writer. Every time a book is published, even self-published, that is someone's dream coming true. Someone imagined that book (perhaps with some spark of divine inspiration), worked at it for countless hours, and brought it to life. A book exists where there wasn't one before. It is an act of creation. For that reason alone, it has value.
Books have the incredible potential to change people's lives. Some will be read by relatively few (and yes, that is hard to accept when confronted with the millions of books the writers on the NY Times Bestseller List sell), yet they may matter to that small few. Perhaps the book will matter to only one person - the writer. The one who writes the book is forever changed by it. Is it still worth it? I would answer that with an unequivocal "yes."
Every book has value. It simply isn't possible to have too many.