I like to have a second rope used as a belay backup to a rappel rope when ever possible. Even with experienced users.

If you have the knowledge and experience, a dedicated rappel rope can be of the
static variety, which can save money and often will wear and perform better at it's
task than a dynamic climbing rope.

Examples.

http://www.amazon.com/Edelweiss-Static-Rope-11mm/dp/B0045E5U8O

http://www.greatoutdoorsdepot.com/edelweiss-low-stretch-static-rope.html

If you are in any situation that might include a fall, a dynamic rope is critical
however.