"he used dadgad for sailors life and although he liked the sound he found it confining and never felt comfortable using it as he could only play in the key of d"

See my previous post about DADGAD in G and A positions.

I don't doubt that standard tuning is the most flexible, and I wouldn't use DADGAD if I were trying to play jazz. It's just that standard doesn't give me the sound I like, to accompany traditional songs and dance music.