Why DMA instead of PIO in GM ?

Hello:
    I've seen in GM document that GM uses DMA to transfer messages from user space to board SRAM. As is known that PIO transfer for small messages achieves much better performance than DMA, then why GM uses DMA for all messages ?
    Compared with GM, many other user-level communication protocols such as BIP use PIO for small messages and DMA for bigger ones, and so it achieves less latency for small messages (about 4.3us for BIP, and 9.6us for GM ) and better bandwidth for bigger ones. Then what's the consideration of GM just using DMA ?
    Any reply would be appreciated ! 

Best regards
Geoge.