Is
this what you're referring to ? Seems a lot of younger people nowadays hold their pens like that. Writing all day at school is tiring and perhaps they do that to relax their hands.
Ahhhh .. but I think I see what you're referring to. In calligraphy you hold the brush vertically. Perhaps sometimes Japanese people also hold their pens in the calligraphy way. When you hold the brush vertically, it means you use the strength of your whole body from the waist up to move the brush, even for a small tiny little hook. That's what my teacher taught me anyway.
They teach you to train this by drawing long lines with the brush on newspaper, horizontals, verticals, diagonals and then big circles, all by moving your upper body. If you do it with the hand or arm alone you have to bend your wrist when you draw a line towards your chest, and it is not so easy to draw good straight lines. It also gives more "expression" into your calligraphy.
That kind of grip is different than holding a pen, it's a bit like you join the tip of your fingers and let your hand fall down from the wrist. I don't think you can use that and draw properly with the upper body and a pen. I don't think the "monkey grip" is a good alternative either.
Basically I think you don't have fine control with the "monkey grip" so whether for kanji or not I don't think it helps write Japanese, like I said probably relieves tension in the wrist when you write a lot.