the disc drags are very nice to have if you are fighting fish in the salt water or big fish in rivers. They are also helpful for less experienced people so they dont try to use their hands to add extra pressure to running fish.
I would never fish any big saltwater game fish without a disc drag.

BUT this is about freshwater as was specified, so here goes.
I would like people to use them for steelhead to that they dont extend the fight by having a poor drag, but you dont have to have one. Most trout I catch, i never even bother to put on the reel. It takes a pretty big trout to need to reel it in as opposed to strip, so i dont think you really need the disc for trout.