BigJim, my friend, I think you're confusing a custom gun with a target specific gun. For a skeet only gun, I think you'll find that shooters with screw-ins don't change their chokes after they get them dialed in. As I said earlier, if you don't need to change chokes, then changeable chokes are just something more to worry about.
Another factor is cost: Even in an upper scale gun, why pay extra for screw-ins if you'll not use them? The same applies to the wood, If a custom fit stock is part of the package, why pay extra for an adjustable stock? My guns with adjustable combs have pretty much stayed put since I got them dialed in.
Our friend in Florida hit the nail on the head when he said it depends on the gun's dedicated purpose. You can't play golf with just one club and you don't need a dozen sticks to shoot pool.