Well, to me the answer is yes and no. Despite admiring the commitment and the time some people invest on those challenges, I would like the challenge the idea and know more about other' opinions. So, here's my take...
Design is not art, it's a profession. In reality, it needs to address pain points of certain target users as well as backing up the business goals. With random UI challenges, the designer doesn't have real users and real world problems. Thus, the success of the design cannot be validated.
Random challenges are like talking to yourself aloud to improve yourself in a foreign language. However, you're limited to your knowledge and skill domains.
User flows define the way people interact and do stuff with your apps. Single screen challenges usually ignore the fact that there's a previous and/or next step performed by the user. Therefore, most of the challenges cannot address the overall user experience.
Especially for beginners, you have a bold commitment to constantly push designs, maybe get faster and make a research.
If challenges are published, you get a constant feedback and create a social presence online. However, I'd still argue the quality of the feedback (e.g "awesome" aka Dribbble).
You get to design user interfaces that you had no opportunity to deal with before. It may definitely widen your ability to handle similar real world cases in the future.