They actually aren't all the same. I have, for example, an aluminum plate that has only a 1" slot for a crotch strap. Some plates have holes all the way around the outside, others have only a few. (This can make a difference for securing extra items, or putting on an argon setup.) Slots can be spaced differently, and camband slots can be, too, and this can affect which wings can be used with a given plate. DSS stainless plates have elastomeric grommets in the bottom slots, avoiding the need for a triglide to keep the webbing from sliding, and DSS stainless plates can accept the shaped, lead weight plates they sell, which can bring the plate up to 13 lbs negative (handy for cold water divers). The new Halcyon small pattern plates have the upper slots closer together, so there is no need for little people to cross their straps, as all my rigs are set up. I have also heard that some plates can be poorly finished, so that the slots need to be deburred before they are used.
A plate is not a plate. They are different, and whether the differences matter to you or not depends on you.
A plate is not a plate. They are different, and whether the differences matter to you or not depends on you.