It's great answer in here. And I want to add 'Why Add a element in Maven3'.
In Maven 3.x Compatibility Notes
Plugin Metaversion Resolution
Internally, Maven 2.x used the special version markers RELEASE and LATEST to support automatic plugin version resolution. These metaversions were also recognized in the element for a declaration. For the sake of reproducible builds, Maven 3.x no longer supports usage of these metaversions in the POM. As a result, users will need to replace occurrences of these metaversions with a concrete version.
And I also find in maven-compiler-plugin - usage
Note: Maven 3.0 will issue warnings if you do not specify the version of a plugin.