Dear John
Yes I think the "K.I.S.S." ("keep it simple, stupid") principle wins out again.

The introduction of parallel cheeks brought a requirement to carry ammunition on a limbered gun in a place other than between the cheeks and so we have ammunition limbers. The splayed cheeks allowed a wider box so a reasonably sized ammunition box could be placed there. The parrellel cheeks being only just wider than the breech on the gun was no wide enough for an ammunition box,

It is the chicken and egg here. The cheeks became narrower so you could not have a coffret (ammunition box) or the introduction of the ammunition limber meant you did not need to carry as much between the cheeks.

Also it permitted seats to be placed on the trails as in the Austrian Cavalry Guns and the Wurttemberg Artillery as shown in the Smoothbore Ordnance Journal.

Stephen