As more and more microcontroller-based embedded devices are connected to the Internet, as part of the Internet-of-Things (IoT), previously less tested (and insecure) devices are exposed to miscreants. To prevent them from being compromised, the memory protection unit (MPU), which is readily available on many of these devices, has the potential to play an important role in enforcing defense mechanisms. In this work, we comprehensively studied the MPU adoption in top operating systems for microcontrollers. Specifically, we investigate whether MPU is supported, how it is used, and whether the claimed security requirement has been effectively achieved by using it. We conclude that due to the added complexities, incompatibility, and fragmented programming interface, MPUs have not received wide adoption in real products. Moreover, although the MPU was developed for security purposes, it rarely fulfills its designed functionality and can be easily circumvented in many settings. We showcase concrete attacks to FreeRTOS and RIoT in this regard. Finally, we discussed fundamental causes to explain this situation. We hope our findings can inspire research on novel usage of MPU in microcontrollers