Optional, but can be heavy if you do decide to take them. Photocopies of useful pages (or those torn out from the book) can be taken instead and are a lot lighter. You can always buy or borrow these on the way from fellow travellers, or get city maps from tourists offices, or even look up the information in an Internet cafe.
You can also get maps on your phone these days, and this combined with a GPS (on the phone if you can get it), makes it a lot easier to get around strange places.
Some companies, such as Lonely Planet, also make audio versions of their guidebooks for most countries in the world plus the major cities, so you can put these on your phone or iPod too. With the right phone you will never have to carry a book with you again (apart from those you take to read, although I listen to audio books a lot anyway).
