In one of my college screenwriting classes, we were assigned the task of adapting short stories into a short film scripts. We didn't own the rights or option the stories or anything - but like you said, it was just for practice. I think for people who only have experience writing prose (which is how most of us begin, I would think), it can be a good exercise in thinking about the differences between screenwriting and prose, and the restrictions and opportunities of the screen. So I think that if you want to tackle an adaptation as practice, go for it. One of my favorite quotes about writing is from Lost's Damon Lindelof, who maintains that you must write a LOT of bad or "practice" scripts before you start writing great scripts: "I got hired as a professional writer for the first time when I was 28 or 29, and I literally have thousands of pages of s***," he says. "A lot of people aren't willing to write s***, or they write two pages of s*** and then they stop. You have to plow through it."
The caveat here is that you have to accept that this script can only be practice because you don't own the rights to the material. If you want to be able to send it to people to launch your writing career, you need to work on an original idea. John August has blogged a lot about adaptations, so I would check out this post and some of his others on the subject.
Also...should you try to come up with your own ideas? Yes! All writers will need original ideas to work on at some point. You can start with the person, the situation, the conflict, the setting, the theme, or anything really. What interests you? What do you want to find out more about?