While many people consider all LiOn batteries the same...in reality, there are lots of different chemical formulations and construction schemes that can be called LiOn. Combine that with sophisticated heating/cooling that the i3 has of its battery pack and a pretty sophisticated charging logic, throw in that they don't let the battery be charged fully or depleted (based on temp and condition), I'd believe the BMW engineers...charge it.
If you read the free i3 e-book, it has some interesting discussion of this. One other thing to consider...recharging the car 10x from 90% (down 10%), is the same wear on the batteries as once from 0%. Now, take a typical cellphone, and this is not true for many reasons...different chemistry, different charging logic, lack of cooling/heating to get the batteries in their optimum state.
So, while it may be true for other brands...trying to use that same logic on a different system is not accurate...charge the i3 up, and don't worry about it. On others, follow the manufacturer's recommendations. Tesla, for example, uses thousands of commodity cells in their battery pack...BMW uses 96 cells. Yes their battery pack is larger, but if you scaled the BMW pack up to the same capacity, it would still be hundreds verses thousands on the Tesla. Even when manufactured on the same line, over thousands, there will be variations in the cells and trying to monitor their individual states to balance everything out is much more problematic verses monitoring 96. They are not the same animal.