I would have to say its is absolutely necessary, unless maybe the cars is new....Polishes and cleaners remove dead paint, light scratches and swirls. This is particularly important when using synthetics, as Anthony said, some work more efficiently with there production partner, i.e. Klasse Twins, Blackfire, Platinum.....These products dont contain cleaning compounds, and/or if any, its very very minute. Clays do remove contaminants, however, it does nothing to renourish the paint. A good example of this is 3M's Swirl Mark Remover. You could consider it a light to med cut, which also has light filling capabilities. Its unbelievable how it rejuvinates the paint. And Im not just talking about light swirl removal, it actually makes the paint come alive again. There are other products that do the same, Im just familiar more so with 3M's SMR. I wouldnt even consider using a synthetic without polishing 1st. Even if it was a brand freeeeekin new paint surface. (But thats my method of madness)