The EPA in the USA requires some minimum detergent in gasoline sold here (that may differ depending on what country you live in). It is up to the individual company to decide if it wants to provide more. How you drive (lots of short trips, only highway, mixed) can make a difference on how effective the detergents that are in the gas can work. So, your results may differ. What isn't known, is if with a certain driving style, switching to a fuel on the Top Tier Gas list would solve an existing problem.
As I read the test procedure, they take the fuel w/o detergents, run a test engine until most of the injectors fail. How long that takes, I don't know. Then, after replacing the injectors, run that same engine in the same cycle, and the injectors must remain clean and working once the detergents are added.
How sensitive any individual engine is to these effects, I don't know, but the corporate word is if you choose a fuel from the Top Tier Gas list, you should have fewer, if any problems maintaining a clean internal engine.
I don't think BMW, along with the other big manufacturers in the consortium, has any stake in the fuel companies; only that their cars run without problems. WHen an injector fails, it could either be an electrical problem, or clogged. But, trying to run dirty will affect overall performance, economy, and longevity while it is compromised. Just like we try to keep the oil clean and working, I feel it's good to try to keep the rest of the internals at their peak...I'll take their suggestions...they have many more engineers studying this than I could hope to cover. The inconvenience of one service call, even if BMW pays for it, is worth seeking out a supplier that is recommended if it has any reasonable expectations of avoiding a problem (to me, it does).
BMW has millions of miles of test data where they spent the time to analyze what characteristics their engines work best with in oil. You don't have to use Castrol, there are others available that meet the requirements. If you keep your car only during the lease period, you probably would never notice a difference. If you keep a car until you're tired of it (and with me, that's typically 6-8 years), longevity is a factor, and again, I'll take the manufactuer's testing results and choose an oil that meets their specs.