SOLR-1352 introduced a multi-threading feature for DataImportHandler. Historically, this feature only seemed to work in a limited set of cases and I don't think we can guarantee users that using "threads" will behave consistently. Also, the multi-threaded option adds considerable complexity making code refactoring difficult.
I propose removing "threads" from Trunk. (But keep it in 3.x, applying any bug fixes for it there.) This can be a first step in improving the DIH code base.
Eventually we can possibly add a carefully though-out "threads" implementation back in.