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JUnit4Migration
With JUnit 5 alpha being already available (http://repo1.maven.org/maven2/org/junit/) it seems like it will drop support for JUnit 3 APIs. Once JUnit 5 comes to JDT it would be nice not to have 3 different style of junit tests around. This work will have to eventually be done to keep up with the newer JUnit releases.
Also http://junit.org/junit5/docs/current/user-guide/#running-tests does not even mention the possibility of running JUnit 3 tests.
The APIs are cleaner in most cases and most developers are more used to 4 API rather than 3, and that is always good for the project in terms of potential contributors.
JDT will not add support for JUnit 5 if that would mean dropping support for JUnit 3 tests, and Eclipse projects are currently not required to convert their tests. Converting tests may not be trivial for some tests, and any contributions need to be verified carefully. See bug 487095 for examples of troubles faced. Green tests doesn't mean the conversion was done properly.
Having as many projects prepared for JUnit 5 release (without JUnit 3 support) would ensure easier migration when JUnit 3 might no longer be an option for the project.