In many (several) of these believed pilot suicide instances, the perpetrating pilot appears to have taken some effort to mask what went on from investigators. This has taken the form of pulling CVR circuitbreakers, shutting down all comms systems, heading for the deepest Indian Ocean, etc.
This suggests that any action that ensures that the perpetrating pilot will be revealed beyond all doubt is likely to dissuade this pilot from making the attempt. Thus, a FA in the cockpit, while perhaps not trained to counteract a rogue pilot's rollover effort, is still likely to yell/scream what is going on, etc. By having all of this captured on the CVR, rather than silence, ensures that the perpetrating pilot and his/her family will not be able to evade condemnation/embarassment, etc. I suggest that this knowledge is likely to dissuade such dingbats from this form of suicide attempt.