A metaphor that is communicated in an elegant way distracts the conscious and activates an unconscious search for meaning. So, if you really want to affect your target you should use metaphors as often as you can.
Because when you do, your target has to decode the message in order to understand the content. And it is while the message is decoded that a little seed of consciousness is planted – a seed that can grow to a big, lush plant if it is watered properly.