The Low G tuning is used on the ukulele. The ukulele has 4 strings which are tuned to GCEA, but the standard tuning rather than having the strings going from Low to High, has the G string higher than the C string, which gives it that true ukulele sound.
However many guitarists decided that they couldn't be bothered to learn the proper way to play ukulele, so they put the G string an octave down, making the tuning similar to that of the 4 highest strings of a guitar (DGBE, which transposed up 5 semitones becomes GCEA). This means that any ukulele specific finger picking techniques can no longer be used, and the same can be said for any melodies that make use of the fact that the G and A strings are only one tone apart.
So anyone who wishes to use low G tuning might as well play guitar! You get two extra strings! Low G tuning goes against the principles of "uke attitude" and therefore should be banned!