Everyone has his own view on the subject and will not change it from what he reads here...
The FMA in the Boeing airplane is meant to show each pilot what the status of the A/P and F/D is, in real time. It is located in front of each pilot and prominently displays this information, highlighting changes for ten seconds. It has been designed to do this effectively and without ambiguity. So why the need to make a callout? How can the system possibly be improved this way? If a pilot is not aware of what is happening right in front of his eyes, and if he does not know that he can check the status of the system at any time merely by looking at the display, how does a callout help? If he needs such a callout he should not be in the seat in the first place.
Of course callouts are important when a switch is moved or mode changed by one pilot, but the callout should relate to what is being done, not how the FMA responds.
Safer flight will be achieved when the pilots actually monitor what the airplane is doing (basic flight and engine instruments) and not the FMA, which only shows what the airplane is programmed to do (should be doing).
If the airplane is programmed by the pilot to fly into a mountain for example, monitoring the FMA will not save you, in fact it will faithfully show that the program is correct and give a feeling of false confidence. Less fancifully, if the airplane is making a LOC capture, the FMA will show that capture has occurred, but only raw data monitoring will tell you if the capture has been made incorrectly, or if the airplane has flown through the LOC without capture, etc. A failure of the airplane to achieve a parameter programmed (does not level off when it should, does not capture LNAV etc) will not be displayed by the FMA and many pilots, accustomed to allowing the FMA to make all their decisions for them, will not see this because nothing will happen in front of them.
Many pilots are swamped with information and overwhelmed by the amount of work required to safely fly the airplane in busy environments, and forcing them to concentrate on the FMA, which is only a switch position indicator after all, prevents proper monitoring of actual airplane behaviour. I have seen many IPs berate the FO for missing a callout, equating it to the worst sin possible, such that new FOs will put every effort into these calls, ignoring all else. And one of the funniest sights is a FO who is PF, still making the calls! If this does not show how absolutely ignorant this policy is, nothing does.
I can only assume that those who need/require FMA callouts are simply ignorant of the purpose of this system, or are ex-Airbus pilots (who are by definition ignorant of the system).