Herein we describe a variety of benefits to be had via the judicious application of DuctileJ:
Let code get out of date: When you're prototyping, things change quickly. You're interested in experimenting with one part of the code and don't want to have to comment out or bring up-to-date other parts of the prototype code. With DuctileJ, you're only bothered with problems in code you actually execute, which is usually the code you care about.
Defer exception handling: You often want to defer error handling while you're prototyping. Checked exceptions force you to either write a place-holder catch clause, or declare that you propagate checked exceptions all the way up to some top-level class. With DuctileJ you don't have to worry about checked exceptions until you're ready to think about your error handling approach.
Test as you go: Sometimes you have to do a big manual refactoring, and you find yourself lost in the woods for hours or days without the ability to compile your code or run your tests. With DuctleJ you can test as you go: do a little bit of your refactoring, run tests that exercise that code, see exactly where they fail, fix those bits, and repeat until that test executes successfully. Lather, rinse, and repeat to guide yourself confidently through large manual refactorings.