DAC's like computers have a 3 year lifespan until they are superceded. However there would be nothing to stop you from liking a DAC you like today in 5-10 years, just don't fool yourself into thinking it will be the equal of what would be then SOTA.

Are DACs really worth it?

Yes, using the DAC in your receiver will give you great audio from your CD transport. Although theoretically the coax and optical connections should be the same, the optical cable is generally considered to be a cleaner connection as it electrically isolates the two components.

Can you leave your DAC on all the time?

ShawnC. My DAC is always on, as recommend by the manufacture (Bel Canto), draws little power when not in use. I leave the PC on all the time so I can stream music to my phone at work or when out and about.