The point has been made already but, to reiterate it, one of the main advantages of DADGAD (apart from the greater depth it adds to the D chord - a chord that crops up very commonly in accompanying fiddle music) is that it encourages a range of chords with no thirds, i.e. neither major nor minor. To my ears those chords sound appropriate when accompanying the kind of modal tunes that you find in English folk song and much dance music. Of course it is possible to add the thirds, and deliberately warm up the sound of the chords, but you don't have them imposed on you as you do with standard tuning.

It's also very good if you like the sound of a drone - playing partial chords of G, C, Bm, Em, etc., whilst leaving D and possibly A strings open.

The main problem with it is that it can become very monotonous if you just play in the key of D and hammer away at the basic one-finger D chord. There are half a dozen of more versions and inversions of D and G chords, all of which give different flavours, and playing DADGAD in the key of G or A modal also gets away from the cliched sound.

Howard Jones is quite right about American music - the standard-tuned guitar has been a part of that since the first commercial recordings of early country and bluegrass music (blues slide gutarists used open tunings, not DADGAD), and DADGAD tends to sound inauthentic when added to a country song or Southern fiddle tune. Though to my mind it can sound good with a modally-tuned banjo or modal Appalachian ballad. There's no comparable history of guitar accompaniment to English folk music predating the 1960s folk revival, so the guitarists experimenting in that period with accompaniments to English folk songs started with a more or less clean slate.