First, let me point out that the OP is asking this question from the Netherlands? (if so, North American gasoline formulation has no bearing on what's sold at the local pump..)
With that out of the way, our gas (in US+Canada) does indeed have a multitude of additives including detergents. This is primarily the result of the Clean Air Act...
As with any product, the intended formulation does not preclude a batch of gas from being contaminated or mixed with something it's not supposed to be mixed with.. (every now and then there's a story in the news of motorists filling up with bad gas at a station or chain.. and not every instance of bad gas gets the press..)
As to harmful effects of fuel additive, some additives can indeed case problems for the emissions system (oxygen sensors usually being the most sensitive parts, followed by catalytic converters..). However, most 'off the shelf' additives sold in auto parts stores are developed to work with such systems and will not cause any problems if used sparingly [and following mfg's directions].. As others have said, there's no reason to use an additive regularly - if you are having issues with gasoline quality, try a different service station...
relevant vehicle: '05 -=C=- T&C Ltd @~120K
Yes Atoman, i'm from the Netherlands.
Before the Chrysler i had a Mercury Grand Marquis 95 and the importer alsoo did the maintenance on our car.
They put every 35000 miles a bottle of injection cleaner in our fueltank.
And yes engine ran smoother. After a lot of miles it's still ran smooth as silk.
But i don't want to damage lines and/or electronics.
While I don't think fuel additives are necessary on a regular basis, I do think that some of them can be useful for some things that can go wrong in the fuel system. Just one case is the fuel sender issue that is common among Caravans and other Dodge/Chrysler vehicles. Parts and labor to instal a new sender would probably be a $400 fix at the dealer whereas an $8 bottle of Techron every 4000 miles seems to solve ethanol-related corrosion problems. Also, I've seen IACV valves and throttle plates that have gotten gummed up and I'm sure that injectors can get clogged from time to time even with today's detergent gasolines. Some stations may have better detergent packages than others that would minimize or possibly eliminate most of these issues but clearly some do not and it's kind of hard to tell which stations have the best gasoline mix.
If the engine is running smoother after using an additive, clearly it is having some positive effect. As to the potential for negative effects, I would only use additives that have a long track record and that have been tested for being O2 sensor and catylatic converter safe.