I doubt that they produce less distortion, whether THD or IMD, especially as SPL goes up. I think the reason why some people prefer them over conventional cone-driver monopoles is because of the smoothness of the polar response... which can be a characteristic of cone-driver monopoles as long as you design for it (and you know what you're doing).Lowest distortion, more accurate to original waveform, etc.
That's why cone/box speakers are called "distortion boxes".
I've never heard of box speakers being called distortion boxes.