If the aircraft is in severe turbulence then overspeed/underspeed protections are an irrelevance as the aircraft will be in and out of both continuously.
This is a key point, since if the turbulence is
predictable and the continuous underspeed / overspeed problems are predictable, it can be time for an advisory / warning, I think.
At different altitude the difference in speed between underspeed and overspeed is different, at lower altitude, with gusts, turbulence instead of constant underspeed / overspeed (and all the risks associated with it) you can stay in safe speed range.
If I know well, the risks of turbulence / gusts can depend on weight and altitude (since the difference between underspeed and overspeed is bigger). And if there are risks (coffin corner aviation + weather + icing) some "early warning" could be helpful.
With reference to the Air France accident I don't really see how control law issues would be a factor.
If I remember well there was an accident where the pilot flying tried to use rudder extensively in a turbulence. One of the early acars messages are related to "RUDDER AND PEDAL TRAVEL LIMITING ACTUATION". I am not sure if operation of rudder travel limiter is different in normal / alternate law, and if unrelaible speed data and switching to alternate law could lead to unsafe operation of rudder. If I read well (elsewhere) rudder travel limiter would work differently in such scenarios.
This, alone, isn't important.
But if we see icing, turbulence, and when some instruments are lose the pilot flying is surprised by "unusual behavior" of the rudder, and as he tries to recover he loses some altitude, and descends into a CB they tried to avoid... Now that is flying in a CB with many instruments failed.