It's all in the chemistry. Many of the compounds in those additives, such as zinc dialkyldithiophosphate are already in motor oil, so you're just adding more of a good thing, that can lead to deposit formation on your valves, spark plug fouling and deterioration of catalytic converters.

Some of those additives may "help" motors that have ALOT of miles and have low compression -- like a "band aid". But I don't see the need for their use in healthy, well maintained engines.