ACARS had been disabled BEFORE the final voice transmission. So it seems to me that identification of the voice making the final transmission will greatly assist in steering the investigation towards the person(s) responsible. The hijacking had already commenced.
There's been much discussion regarding the wording of the final contact. If one of the pilots was up to no good and wanted to disguise the fact and leave as much confusion as possible, using a signoff that's non-standard but not odd enough to cause immediate concern would be a simple way to do it.