Long Sensei,
Joshua-san is a student of mine here at Nevada Budo. I believe his curiosity about grip strength is not only about holding the sword. Personally, I train my grip strength quite often. I have found that my technique has improved because of this training. The way I see it, the better my physical fitness, the better my technique (in many aspects of budo) will be able to become.
Respectfully,
Ron Taniguchi Jr.