The benefit of a life jacket though is that it can be put on by passengers (uninflated) during the descent (possibly with assistance from cabin crew or other passengers). They are then in a position to evacuate immediately.
When using the seat as a floatation device then the passengers have to wait until they have ditched then unbuckle themselves, turn around and remove the seat (disorientated and probably in a broken aircraft) before they can start to evacuate.
If the authorities are happy, then fair enough - the airlines are doing as much as they need. But I don't think that a seat floatation device is as safe as a life jacket.