I don't think it's silly. I think it all goes to how a pond is constructed and functions. I think overall volume matters more in terms of water quality, with surface area being a big consideration. Less space? Sure. Less depth? No. And a two foot deep pond can run aerators just like a 3.5 foot deep pond, so no difference there.
Why not go a foot or 18 inches deeper? Because it can be back breaking labor. Or next to impossible in areas with bedrock. And if it's an option NOT to, then why not mention that it's ok to have a 24 or 30 inch pond, too?
Why not go a foot or 18 inches deeper? Because it can be back breaking labor. Or next to impossible in areas with bedrock. And if it's an option NOT to, then why not mention that it's ok to have a 24 or 30 inch pond, too?