I think that Java is now popular in web based application development and mobile application development in industry. As I've seen in many vacancies, they asked for J2EE or J2ME related qualifications. But I didn't see any vacancy which is asking about Java SWT/Swings related qualifications.
Is it worth giving if I learn about swing GUI development? Is it helpful for me to get a job in future, if I have experiences on swing/GUI development? Well, I'm working in a small company right now. They asked me to complete a lot of Swing/GUI projects, which takes more than 6 months to complete. If I considered to join on these projects, can I earn anything else (valuable experiences) other than money?
The primary benefit for Swing (and SWT) is for applets and desktop applications. However, applets didn't become as popular as originally expected (if Microsoft had bundled a real Java with Windows, the story might have been different). And web interfaces are currently more fashionable than desktop apps.
There are some good Java GUI apps, such as the design tools from Pentaho, the ArgoUML/Poseidon UML application, FreeMind (a personal favorite) and others - including Eclipse. For that matter, I've got an open-source project of my own that's based on Swing if I can just get some people to help out (it's a Visio-style object-oriented graphical design system).
There is a market for people with SWT and Swing. It's just not as large as the web app market.
Customer surveys are for companies who didn't pay proper attention to begin with.