Why is Denver International Airport runway 16R/34L rarely used?
KDEN has a lot of runways, so they have the option of not always using some of them. As FreeMan pointed out, 12,000 feet is plenty for just about any aircraft flying in or out, even at 5,300 feet elevation. Being such a long runway, it may also be good to leave it open so that no aircraft need to be moved in case an aircraft declaring emergency would need the extra length. Runway use will depend on the wind and traffic, so depending on when you are observing, you may not often see conditions where it is used.

North flow: arrivals on 34L could interfere with runway 7/25, and for departures it is further from the terminal than 34R. TomMcW also points out potential noise abatement concerns.

South flow: departures on 16R could interfere with runway 7/25, and for arrivals it is further from the terminal than 16L.

Even with 17L/35R currently closed it is not always necessary to use 16R/34L. 8/26 and 7/25 can be used for east/west departures and arrivals, and 17R/35L can be used as well.

Runway 16R/34L can still be used as a secondary runway though. It can certainly be used as 16R for parallel approaches with 16L, or departures as 34L with approaches on 34R.

To put things in perspective, Denver has more runways than both KLAX and KATL, which see more aircraft movements.

Also, while far from authoritative, the virtual Denver ARTCC lists 16R/34L as a secondary runway in all situations.