The reasons for a capture card are both for capture as well as export. If you wish to capture "live" uncompressed HD, you can go from HVX (or even the HDV cameras) component to AJA converter component->SDI and capture directly to your supported NLE. The Xena also allows you to direct your output to SDI for monitoring on SDI monitor (or even DVI with their SDI->DVI converter) or to SDI deck. I am on the Mac side and more familiar with the Kona line rather than the Xena, but the Kona 3 would give you a real time upconvert should you wish to go directly to HDCAM deck via SDI.
If you are just acquiring via USB2 (which you would do on the PC side rather than on the Mac side) and then delivering to web, DVD, or are delivering on DVCPRO HD tape via firewire, then you don't need the capture card. It still begs the question, however, of how are you going to monitor your HD.
