I buy Blue Guides because of their in depth coverage of museums, galleries, churches, historic buildings and for the detailed description of the gems that are to be found within; they often contain things which more popular travel guides omit, show little detail or dismiss.
For example the Blue Guide has 10 pages on Leiden compared with the 5 pages in a rougher guide I took with me but more importantly the rougher guide did not mention the Museum Boerhaave which is the Dutch National Museum for the History of Science and Medicine. It has many scientific objects from the Dutch ‘golden age’ including the first pendulum clock by Christiaan Huygens - well worth a visit.
The Blue Guide has almost a page on Teylers Museum (the Netherlands' first and oldest museum, open to the public since 1784) whereas another guide has just a paragraph.
Lastly if you rely on other guides you might miss the three 17th century model ships in line hanging from the ceiling in the Grote Kerk, Haarlem.
Although Blue Guides may be out of print, the locations they cover are likely to be around for a long time.