@BenF: you are right, I alread noticed that I could do this. But the whole point was to get a simple solution. Addind more parts would make the whole setup unnecessary complex (especially: increase the number of solder joints). In addition for me it solves the wrong problem. I am driving a backlight. So I do not want to regulate current but "perceived brightness". Something that can not at all be achieved by a constant current regulator. So I will keep it simple. I did not even put a diode in because the LEDs are "the diode" 
With regard to efficiency: I did not yet measure to much but running full tilt the setup consumes an additional 90mA@3.3V or ~300mW. The CPU + LCD alone consume right now ~6mA@3.3V my old setup consumed 110mA@5V and I could not regulate the backlight. At 30mA@3.3V I still get very reasonable brightness and only 100mW power consumption so right now the setup is good enough. And most important: the backlight does not flicker since I am always way beyond 10kHz If I decrease brightness further current consumption goes down as well.
Udo