Lets get it clear, once and for all time. Submarines of whatever color have never been designed for doing what is required in these circumstances.
The French sent L'Emerauld (a nuclear sub) to aid in the initial search for AF447. The first problem that had to be addressed was that their sonar detection was not aligned to the ULB frequency of 37.5kHz. That with some software updates and testing on another Rubic class sub was achieved. Probably too late in the calendar of events.
So, other than making use of their sonar, providing it can address the task, subs are of no particular benefit when comes to searching for aircraft that have become seacraft or any such like UFOs
I think you understate their potential. Most of the equipment currently involved is not designed for what they are being used for in this search.
Modern military submarines have extremely sensitive passive sonar system with direction finding capabilities, so much so that those capabilities are highly classified. Whether the U.S., for example, would allow such resources to be leveraged or if they could even be placed in time is another question.
Ultimately, the decision by any government to deploy a submarine indicates there is at least some thought that the platform might be useful, so the current activity belies your overbroad statement.