.... OH! I forgot, this always happens.. we're playing some fast rockers one after the other and the crowd is dancing up a storm and they decide to either go on break or play a slow song... = loose the crowd again. Ok, I'm done ranting.
If you are playing a concert, I agree that this is not good. On the other hand, when playing a bar gig, it is often necessary. You have to remember why you are there. You are primarily there to sell drinks; entertaining the crowd is secondary. On more than one occasion, bands I have played with have been asked to kill some stage time or play some slow songs, because the people were dancing too much and not buying drinks. We usually used this time for an impromptu commercial urging the audience to patronize the bar and tip the waitresses.