The US seat cusion thing is mainly in response to the fact that there is so much domestic flying and so little open water.
I think the response by the OP comes down to the fact that there are very few ditchings that occur. In a pre-planned ditching, however successful or not, there is a chance that you survive the impact. Regardless of your chances thereafter, a life jacket would help to some extent in keeping you afloat. The pre-planned scenario is pretty rare due to the reliability of aircraft. Unplanned ditching usually means a crash.
There are however the cases where aircraft over run or crash just prior to landing where water surrounds the airport. In a pre-planned emergency the crew may find it prudent to prepare for a ditching aswell or in an unplanned scenario and at that stage of flight there is a pretty good chance that alot of the fuselage will remain in tact and there MAY be time or a passenger's presence of mind to don a life jacket.
One passenger cited the Ethiopian hi-jack example where many inflated their life jackets prior to impact causing deaths. Passengers are always told not to but because many don't listen couples with the chaos of the situation some passenger may indeed inflate it. So whilst you can categorically say that early inflation of lifejackets in the Ethiopian example impeded on peoples chances, some of those that did survive may have survived because of the lifejacket inflated after impact at a time when injury meant they couldn't easily stay afloat.
In conclusion, I don't think lifejackets are particularly useless but that they are rarely required. As someone already said, it would take a brave airline to remove them.