I think liberal licenses or weak copyleft are appropriate license for certain software, usually libraries. What we see today is that us developers have great freedom in what free libraries we can use to build our applications from. However, that freedom ends with us. It never reaches the users. We use liberally licensed libraries to build proprietary end-user applications. The GPL aims to ensure that freedom reaches the users. People say that the GPL is less free than liberal licenses because it "forces" developers to give source code to users, but the freedom to restrict others is certainly not freedom.