Put 10 audiophiles in a room to discuss biwire, and they will emerge with at least 11 different theories.
The majority of speakers today have biwire connections, however only a minority have been specifically designed for this from the ground up. For the rest it's just an add-on to maintain audio fashion. The manufacturers of the speakers that are specifically designed for biwire only recommend using two separate runs of identical cable for the biwire. So it appears they think it is necessary.
I agree that changing cables can make a difference in tonal character of the sound, however IMHO it doesn't make common sense to try and change high and low inputs on a speaker separately. That could cause an imbalance of frequencies at the crossover point.