There are a multitude of reasons why amateur data is important. I myself have been participating in that partnership. Amateur data provides a wealth of astrometric and photometric data on the comet being studied. Over the course of the mission this will also allow changes in the comet over time seen by the ground to be correlated to the data collected up close by the spacecraft. Such a long term and intense observing campaign can be conducted without any taxpayer money at all by amateur astronomers like me. That's a good thing, not a bad thing.