I know what you mean - and I firmly believe the ebb and flow of energy during a song - particularly during solos or transitions between differing parts of the song - provide opportunities for provoking the subtle awareness changes in your audience that something is happening musically. The cadence of the drummer whether he is leading the intensity change or following another band member's - seems to be a trigger for this phenomenon. I notice (as a jazz player) - that such changes often brings an opportunity to spark a song to greater heights or a spectacularly creative passage - at least in our own minds (ha) if no one else's. A drummer - whether canned or live - who has rhytmic interest and variation - sure takes a load off the piano player.