What is the CSLP? Is membership in CSLP required to host a Summer Reading Club at a public library? What are the benefits to being a CSLP library - or not being one?
Question 1: The Collaborative Summer Library Program is a group of states that use the benefits of being a large group to create and implement a summer reading program that is low cost and high-quality.
Members of the group (including: 50 states, the District of Columbia, America Samoa and Mariana Islands, along with the Cayman Islands as a Provisional Member. All public libraries in member states are included in the memberships) send two representatives (in my state, they are the heads of Youth Services for the state's library system) attend annual meetings and vote on themes, illustrators, vendors, and more. Every individual who belongs has the opportunity to voice their ideas & needs to their representatives, who bring these to the wider group.
The CSLP votes on the themes several years ahead of time in order to secure artwork, create manuals (with both rules & regulations, particularly around copyright, as well as member-submitted suggestions for programs, crafts, contests, decorating, activities, reading lists and more), sort out vendors and otherwise organize it.
Question 2: No, you do not have to belong to CSLP to run a Summer Reading Club. Libraries have run individual or regional summer reading programs they design & execute themselves. You do have to belong to CSLP to use the copyrighted artwork, however, and the other benefits outlined in answer to Question 3.
Question 3: As above, the copyrighted artwork is one benefit. For example, children's illustrator Brian Lies created images & posters to be used with the 2012 Children's theme. You also receive the manual (passed out in CD form with the artwork on it where computers are common; in hole-punched binder-ready style for those without computers), there's an online forum, themed incentives & decorations (the cost of which is much cheaper given the bulk numbers the size of the CSLP can pull in), and the time savings not having to come up with a unique theme every year. In my state, the state library system provides some of the paper goods for free (subsidized by the state's sponsors, a state sport's team): posters, bookmarks, reading logs. Oh, yes, and the CSLP comes up with related themes for Children, Teen & Adult Services.
As for the benefits of not belonging or participating, you would have the ability to create your own theme (could be a pro if you like the creative freedom or don't like the theme the CSLP uses). I'm also assuming that there would be some cost savings for the state if they didn't belong, but given the above benefits (to all public libraries within the state), it is unlikely that those savings outweigh the benefits.