I think disc brakes are fairly valuable on dirt. With rim brakes, you get moisture on your rims by crossing a stream or just morning dew, then dirt will stick to your rims, then it grinds when you brake. That grinding was very disturbing to me.

On long descents disc brakes are better because they take less hand pressure.

If riding on mostly dry pavement and not doing much long descents, then disc brakes aren't as important.