Reason #1 (practical): By definition and necessity, hate machine run on pure propaganda. In order to peddle pure propaganda, you have to have established the perceptual history to make it work. The right enjoys the existence of this history (see, e.g., Alex Carey's _Taking the Risk Out of Democracy_). We do not.
Reason #2 (ethical/long-run practical): Two wrongs don't make a right (witness USSR, Israel). The left makes advances in history by building up movements of truth and love (witness Gandhi, MLK, most of the Latin American left). It creates embarrassment for itself when it turns to propaganda and sheer shoving matches.