I'd value HDMI input because that would enable the Voodoo glide accelerator to play with any new PCIe main GPU.
But a HDMI output needs explaining.
The thing is, the original 3Dfx Voodoo feeds its digital signals from RAM to RAMDAC (Digital-to-Analogue Converted) for analogue VGA output. The "problem" is that any RAMDAC adds some noise, and fuzziness, which is why so much money was put into using high spec DACs (particularly in high resolution pro workstations).
One possibility is that the added HDMI is being fed from the RAMDAC by converting the analogue signal back to digital via a ADC (Digital-to-Analogue converter). The "problem" with that is any ADC adds some noise and fuzziness. I'd be concerned that fuzzy*fuzzy=fuzzy^2.
Alternatively, the added HDMI could be tapping the signal before it hits the RAMDAC, which would be okay but.. the passthrough from the main GPU is analogue, so that is broken or fuzzy.