BMW recommends the 15K mile intervals because it saves THEM money since they pay for the first 3 years of maintenance. That said, 3K on synthetic oil is overkill. Even if you drive short trips you should be able to go 5K between changes. The more you drive on the highway, the longer you can extend the drain intervals. Even doing driving schools I have taken advantage of extended drain intervals. Oil analysis has backed up my use of the oil in this way and has proven that I haven't harmed my engine at all. In fact, all wear metals came back below average.
One VERY important thing that most people seem to overlook is that proper filtration is just as important as a high quality oil. Someone here asked of the filter should be changes also when DIY between dealer services. YES. Fresh oil that is passing through a clogged or partially clogged filter will build up contaminants much faster than if you change the filter.