That's depends very much on whether you plan to be using features like Audyssey's MultEQ XT32 and/or bass management. Many and arguably most A/V processors and receivers don't come equipped with analog-to-digital converters (ADC) to convert the analog signal inputs into digital signals. Therefore, if you plan to use MultEQ XT32, bass management, etc. on an A/V processor without ADC, then you have to either use the S/PDIF connectors for stereo (coax or optical) or HDMI for multi-channel so that the processor receives a digital signal in order to be able to apply the MultEQ or bass management algorithms in the digital domain.

We don't yet have a complete picture of the 978's features but up to now, I've seen nothing indicating that it had ADC capabilities.

Furthermore, when you use analog interconnects - unless you have very transparent ones which are usually very expensive and high-end - well your average analog interconnects tend to work as tone controls and you're almost always "tampering" with the signal loosing some nuances (information). The conversion processes, either ADC or DAC almost always incur slight penalties unless using very high-end gear and interconnects. OK, I can already hear signs of protests about jitter in the digital domain over S/PDIF or HDMI but IMHO those have less of an impact than using analog interconnects. The basic rule I would follow is, if you're source material is digital, then keep the signal in the digital domain throughout the whole digital processing chain and then convert the signal to analog at the very end in the DACs. It's also generally much cheaper as good S/PDIF and HDMI interconnects have much lower costs than high-end interconnects.