Absolutely usefull to a boxer imo. Its a great way to gain strength fast without gaining weight and takes very little time to perform a full body workout.

Ross Enamait has a good chapter in his book Infinite Intensity (which i recommend to everyone) and there is also a good chapter in the Bruce Lee book The Art Of Expressing The Human Body. They were a key reason that Bruce could perform such impressive strength feats after only seriously training for strength for a couple years.

I think they became less popular as the whole bodybuilding and gym culture became more popular. After all you dont need any expensive equipment to perfrom isometrics so no one makes any money off them. Also compared to lifting weights, they are inferior at building muscle, so they're less than desirable for people training soley to add muscle.