I've seen this suggestion made before and one of the reasons offered for not doing so, from the airline's point of view, was psychological. For a large number of passengers, passing such a demonstration in progress would have a decidedly negative effect.
I agree with the OP and would like to see a dry run offered. I'm often in the exit row but check out the instructions for that aircraft; a pre-boarding mockup could shave a few seconds off the procedure.