Having any codewords that are designed to be used over the PA in the event of a mishap is a pretty poor idea.
Already the pax are aware something is wrong but likely don't understand exactly what... add to that an announcement over the PA they don't understand and it's guaranteed to put the fear of Christ up them - It's not rocket science.
The use of a code, enables each member of the crew to initiate the action that is relevant to their station and to the team requirement. In situations that might come under the category "emergency," the flight deck crew will normally be very busy dealing with their own priorities.
I absolutely agree that communication with the passengers is very important and desirable, but only when the situation permits that to be done properly.
Codewords enable a lot of basic instruction to be given to the people who need to know, in a few syllables, and without tying up an otherwise important time resource unnecessarily.