I believe they are adding value. They can get you out of the large hotel chains and into small and independent hotels. Their app is very good for tracking your travel reservations (have you seen some of the obscure hotel sites and confirmation systems?). They handle payments in a secure way (I've been asked my CC # AND my code over the phone a number of times by small hotels). And they assist you when things go bad with the hotel. E.g. I have had a refund handed to me by booking after contacting them despite of the hotel refusing to do it initially. Plus, I get a nice discount and a bonus every now and again.
In a world without intermediary sites there'll be just the large hotels that would act as intermediaries. I think competition is good for the consumer.