is there any reason why this wouldn't be better than converting to S/PDIF?
Uhhmm, would not, would ... don't ask those difficult questions !
I'm not sure what you'd want with the I2S connection, because I don't think there's anything to connect it to. Yeah, well, internally, but careful, because this is the most normal for most of the DACs (or CDPlayers for that matter).
In fact it is the other around, similar to how you put the question : would there be any reason to convert USB to SPDIF, while at the end of SPDIF in there it always has to be converted to something else (unless DAC chips exist that can take SPDIF).
So indeed uhhm, not, the TwinDAC+ converts to SPDIF (yes).
Concluded : I2S is direct only if it is used externally (like a CDDrive with I2S output).
I2S can be used as input for the DAC chip(s), so a USB-I2S (internal) conversion would be best. If the DAC chip takes another electrical signal, then obviously it would be best to convert to that directly.
(note : DAC chip can be input buffer etc.).
and buy a dac probably around the time PeterSt puts out XXHE 1.0.
You're saving money, are you ? hehehe