Trivial Description
CompressingCodec was designed as an experimental Codec to see whether we could reduce the amount of disk space required by Lucene indexes.
In order to ease testing we would need to enforce such Codec to be used without changing configuration / index definitions because it would be already too late as the first "version" of the indexes would be created with other codecs (OakCodec, Lucene46Codec).
Since CompressingCodec can't be yet used as default using a system property to experiment with that seems like an ok temporary option until decisions whether it can be used as default or not are taken.