Ability to use PCI devices directly in guest operating systems would be great. Current macs can utilise Thunderbolt 3 ports to connect PCIe devices at 40 Gbps, but there is huge problem with drivers availability for the devices. Connecting devices directly to Windows guest and bypassing macOS would resolve the issue. In my particular case I would connect SAS card directly to Mac, rather then use 10GbE network connected to NAS with SAS card. Another usage scenario for IOMMU is gaming, as described in following request: https://forum.parallels.com/threads/vfio-support.344691/ . PCI passthrough would allow to assign GPU to virtual machine and use it (play games, use CAD software etc.) as if machine had this GPU attached directly in the system, with minimal performance penalty.