Zigbee Transceiver Using OQPSK Modulation under Fading Channels
The pulse shaping is only required if you are concerned with spectral efficiency, as the pulse shaping will convert what would otherwise be a Sinc function in frequency to a spectrum that is much closer in overall bandwidth to your symbol rate. Also if the receiver assumes a specific pulse shape, the sensitivity will be optimized by transmitting with the matched pulse shape (as the matched filter in the receiver will then present the optimum SNR. Other than that, it is not necessary.

See my picture below demonstrating the time domain waveforms and frequency domain before and after pulse shaping. Note that the primary motivation to do additional pulse shaping (beyond the rectangular pulse you are otherwise using, which is indeed a pulse shaping in itself) is for spectral efficiency. The pulse shaping is required in IEEE 802.15.4 ultimately due to meeting regulations (such as FCC in the US) to ensure we all use spectrum efficiently and within regulated allocations and not interfere with users in other spectrum allocations. Observe the dramatic change in transmitted spectrum with and without the pulse shaping filter in the example below!