Because some people annoy visitors with frustrating pop-up ads, visitors use ad-block extensions to save them. Adsense has a feature that allows displaying a static ad or image when the live ads don’t load, generally because the auction was not successful.
I don’t know if Ad block completely disables this feature along with ad, but if it doesn’t, you can use that feature to sell your own stuff like eBooks or recommend any product, when visitors use ad-block.
Hope this helps.
But seriously, this is an issue which shouldn’t be overlooked, because advertisements are the only way to get started in blogging (Affiliates work with increasing good amount of visitors). All we can do is hope someone finds a permanent solution to the problem or develop a new way of advertising.