Exactly right, it isn't economic for whole fleets, or even whole long haul fleets, to be continually transmitting data to satellites.
There is, though, a case for aircraft in areas of consistently poor communication with ground voice stations and HFDL facilities, on thinly travelled routes, having the capability to transmit out of the norm data and the transmitters having enough stored power to continue to transmit at least position once/if the aircraft's own electricity supply fails.
Examples would be, over water:
the South Atlantic, portions of the South Pacific and the southern Indian Ocean and
overland:
portions of the Arctic, parts of Saharan and sub Saharan Africa and parts of Asia in the CIS/Mongolia/China region.