Good art demands time, but good art also demands practice and experimentation. I'd put challenges like this in the "practice and experimentation" category.
• 24-hour (or shorter) challenges may help you identify needless churn in your art process.
• Restricting your toolset (e.g. removing colors or editor-plugins or instruments you're most comfortable with) forces you to learn more about your craft without your normal crutches.
• Creating artificial size-constraints (e.g. Alan Kay's "t-shirt computing") can guide you toward the essence of your art.