The DEUs dont only feed the displays, but they seem to communicate bi-directionally with the EECs, the FCCs and others... My sources apart from the FCOM dont mention much about them either. Just a couple of additional points: The onside FCC will not be able to provide pitch guidance in case of a DEU failure, also the roll guidance will be removed when the APP mode is active, the EECs go into panic mode (well, to Alternate), but instead of taking info from just one DEU they continue to operate without them in ALTN mode...