Why are harmonic emissions from an amateur radio station not wanted?

Harmonic emissions are radiated at integer multiples of your fundamental transmit frequency. Because these harmonics fall on entirely different frequency bands (often outside of amateur radio allocations), they can easily cause severe interference to other communication services, such as emergency services, aviation, or commercial broadcasting. While they do represent a tiny amount of wasted power, the primary concern is the disruption of other users' communications. They do not typically interfere with stations on adjacent frequencies (which is caused by splatter) or overload a nearby transmitter.