The HV battery is never totally discharged if I understand things correctly. Even when it indicates that it is totally discharged and the car stops, it is apparently programmed so that a total discharge to you is actually maybe a 95% discharge or so. That's a built in safety feature. Same thing happens at the upper end. A 100% charge is only about a 95% charge in reality.
What we have seen happen is that a discharged 12v battery causes issues with "talking" to the HV battery. We think that the actual cells in the HV battery are still good, but that the part or parts that stop communication are not individually replaceable, at least by Mercedes/Smart dealers. Thus the need to replace the entire unit.
Again, please take my thoughts with a grain of salt. If anyone has better info or can correct me, please do so.
2014 EV Coupe 19,500 miles
2014 EV Cabriolet 17,000 miles