Top 5 reasons to use Maven 3 (instead of Maven 2) now:
1/ Backward compatibility
with Maven 2
2/ Faster build
Build time reduce 20%-30% compared to Maven 2
3/ Easier to embed
Thus the tools such as jenkins or m2e will support it better (and probably more tools will utilize maven)
4/ Better management of plugins
It will use latest release version of the plugins which we don’t specify the version. Maven 2 use SNAPSHOT version so it is risky. Maven 3 also allow to define <pluginRepositories> besides the <repositories> .
Maven 3 supports some other languages (or at least started doing so). And it will probably be simplified to support POM in YAML or JSON format as a DSL to support those languages.