I think the primary idea was to get rid of the classloading code from class org.hibernate.search.impl.FullTextSessionImpl, see this method:

private MassIndexerFactory createMassIndexerFactory() 

By moving it to a service, we'd take advantage of the plugin structure of Service. In addition, the MassIndexerFactory would be created only once, while it's currently being created on demand on each need.

But we need to consider the OGM needs, so this might be just an horrible idea which doesn't work out, in which case feel free to close this as won't fix. The fact that it's created multiple times is certainly not a performance issue, this is just a matter of refactoring & cleanup of potential dead code.