The biggest benefit of Genuine BMW oil is not only that it is a full synthetic blend, but the detergent package and shear resistance are what enable it to be a "15k mile" oil. The oil is specifically built for your engine, but your engine is also built specifically for this oil and is very sensitive to the molecular size and detergent package of the oil used.
Other oils aren't poisonous to your engine, but will require more frequent changes and closer monitoring. We usually suggest 12-14k and 1year and have never had a problem while continuing to use Genuine BMW oil in our service shop. At less than $8 a quart, its really not a bad price for peace of mind.