I've just joined the BCS *because* the membership includes:
a) full access to the EBSCO library
b) free ITNOW magazines with useful information
c) discounts on useful books
d) many other resources that I can now access to learn new things and advance my career
The post-nominals can also be a valid reason to join because they may give you a professional recognition, but this is not the main point for me. I'm a strong believer in acquiring knowledge from scientific journals, books, magazines (there are some high-quality ones), and other places. BCS provides me with the access to online libraries so I can gather more information than if I used only IEEE and my university library.