Should I be concerned about wearing out the clutch when driving screws with my drill/driver?
This isn't something I would worry about at all. If you are a typically householder you don't use your driver 8 hours a day 5 days a week year after year after year. If you do, you should be buying a tool designed for professional use (which the DCD790 appears to be, judging by it's price).

I'm just a householder and I find drills last me at least 10 years. I still have all the drills I ever bought and they all still work (to some extent). On one of them I can't turn off the hammer-action after trying brute force removal of chuck (I broke it by severe misuse), but the clutch is fine. I bought a pro-grade replacement drill/driver (Bosch blue) at a similar price I paid 10 years ago for the non-pro one I broke. 

The clutch on the driver is essential, I always dial it down to the lowest setting, then turn it up as needed. I'm pretty sure it is designed to be used and shouldn't wear out any faster than the gearbox or motor.