The Argive Grip is the best grip if you intend to use the Scutum as suggested by the traditional representation. That's why modern riot police shields use the Argive grip. That is also why hoplons used the Argive grip.

The Roman's were familiar with the Argive grip. Triarii shields used the Argive grip. Still, Romans chose not to use the Argive grip in their scutums.

This begs the question: if the Argive grip was the best suited for the traditional representation, and, if the Romans were familiar with the Argive grip, why would they drop the Argive grip in favor of the horizontal handle?

Their choice of handle indicates Romans did not mean to use the shield as depicted in the traditional representation. It might have been used in such a manner under exceptional circumstances, but, it was not meant to be used that way.