I'd say the BOV is there to prevent overpressure which leads to detonation, which leads to broken motors. While it's true that backpressure might stall the compressor, I'd call that a secondary purpose.
The wastegate actually prevents over boosting(or overpressure) if you prefer. Since the wastegate diverts exhaust gases from the turbine to hold boost at a constant pressure. Also the primary purpose of the BOV is to prevent a pressure backflow and stalling of the turbo. Since stalling or reversing the compressor turbine is gonna do the same to the exhaust side....