Using REX to maintain SOC kind of works, but after some time you end up with battery running out in longer mixed drive trips when there is no time or a chance to charge. This happens because when you stop the car after highway speed to make a quick pitstop, the REX is usually falling a bit behind the value to be maintained. When you restart the car, your maintained SOC value is then lower than in the beginning of the trip.
When maintain SOC is activated, the marker of the maintained battery level should be for example +5% the current state of charge. This way you would not lose charge because of a quick pit stops.
This should be rather easy to implement and even a one liner code change.
What do you think? Any comments. I say: Please BMW implement this.
If you like to see this implemented give it your vote in the comments.