Consider what causes jitter in the first place, even with the extra conversion level, the signal path is so short (the physical length the signal is going to exist as s/pdif), that unless MHDT really messed their design up badly somehow, the benefits of being able to use USB for getting the signal to the DAC outweigh the possibility of jitter caused by this extra conversion, which would be very minimal compared to using s/pdif over exponentially larger distances (like from source to DAC).
The only reason NOT to use USB, would not be whether it goes to I2S or not directly, but for electrical isolation from the PC... even USB straight to I2S is going to have that problem, maybe more maybe less, but I don't know how sensitive I2S is to that vs the way MHDT does it...
I use a DIP to isolate my PC from my MHDT, I don't have a USB version to compare it to though.