rule question - demigryph monstruous mount

You've said it yourself: Only the rider makes his attack.

you might still want a second rank if you plan on charging them into a situation where you are expecting some heavy return fire or you risk losing one or more models to a round of shooting, magic or terrain tests.

Essentially the strategic reason to keep your demigryphs in a 3x2 formation is so you can maintain the attack output of the front line for as long as possible.

on the other hand having two groups of 3 might give you other advantages as well such as maneuverability or presenting multiple targets and so forth... all offset by having them become more fragile to any kind of return attacks.

In the end the choice ultimately falls on what role you see them having on the battlefield.

Thank you for the explanation guys !