Lets face facts.
There is "nearly" always time to communicate. To send folks "down the slide" without very good reason is to condemn many to unnecessary injuries.
OTOH, lets take SQ006 as an example. Aircraft has clearly stopped, smoke and flames are clearly entering the cabin, panic has set in, no time to communicate....time to get out, NOW.
Just one example, and I suspect the cabin crew on SQ006 saved many folks with their actions.
This clearly NEEDS to covered in SEP training for cabin crew.
To training departments: are YOU listening? Hello, anybody there?