After listening to Irish and Scottish bands since 1980 (29 years), my view is this.

The bands wanted to emulate the loud, driving sound of modern popular music. They needed to do that, because when people pay for a concert, they expect a full, exciting sound. To get that sound, they turned the guitar from a harmony instrument to a percussion instrument. They did that with DADGAD and a monotonous, pounding stroke.

As somebody said above, it gets samey. Boy, does it get samey.

The Irish bands went a step further and changed the drum (bodhran) from a percussion instrument to the melody instrument. If you have had the pleasure of hearing a good bodhran solo, you will know what I mean.

Is it worth it to learn DADGAD? Perhaps. I'm not going to bother, myself. We play 'Celtic' music all the time without it. And we use more than three chords.

Lefthanded Guitar: to learn more about DADGAD, why don't you go to YouTube and search for those lessons mentioned above. That is, if you have broadband, and I hope you do.

There's also a site called Expert Village that might have something.