From a Spey rod users perspective, you almost can't get a reel that doesn't have a 'disk drag' of some sort (Older Hardy's being a notable exception).

The 'drag' created by you line going through the guides provides a considerable amount of force against all but the largest of fish. Or as someone noted above, a relatively large fish whose gotten into fast current.

If I remember correctly, even on level wind reels the recemodation is the drag should be set for a max. of 1/2 of the tippet breaking figure.

My reel drags are really only set to prevent 'over run' if I get ham-handed 'palming the reel.'

Should have added: a 'lot of drag' on a spey rod is almost a guarentee (remember the length of the typical rod) that the fish will go in to a 'jumping mode' due to the lheight of the rod tip over the water. That's when you'll loose most of them unless your quick thinking enough to 'bow to the fish.'