Are oven bags anything more than disposable dutch ovens?

There are differences between baking in a plastic bag and in a Dutch oven. If you have access to both, I prefer the Dutch oven.

What both do is to

• Trap steam
  This makes your food a bit moister, and keeps pan juices and additions to the roast, like a dry rub or mirepoix, from drying out into an unappetizing, carbonized spot. It is not as important for meat as for bread. I haven't tried baking bread in a bag, but maybe it will work.

• Change the mix of heating processes
  Baking in an oven involves heating by conduction, convection and radiation at the same time. The heating elements emit a lot of infrared radiation, which cooks the surface of the food, giving it a nice crust, but does not penetrate it to cook the inside. When you put the food in a small closed space, the material shields it from that radiation. Althought there is some secondary radiation from the shielding material itself, the amount of conduction heating goes up a lot. This gives you more even heating and the inside will be well cooked before the crust burns.

  Be aware that if your bags are transparent or translucent in the visible spectrum, chances are that they let at least some infrared waves through and a Dutch oven will perform better.

Where bags and Dutch ovens differ is that only a Dutch oven gives you a buffer for temperature changes. Its large thermal mass and relatively low heat conductivity make sure that there are no rapid temperature changes when you open the oven. Using a Dutch oven will also help if your oven heats unevenly. As the oven heats the Dutch oven, the heat travels throughout the Dutch oven, and only then it heats the food on the inside.

This gives you a more even roasting process. In a logistically problematic situation, you can hold the food longer in the Dutch oven without it cooling on the counter or drying out in the oven. Remember to allow for residual heat transfer and turn off the heat 4-5°C earlier than usual.

Oven bags achieve a similar thing to Dutch ovens, namely keeping moisture mostly inside the bag during cook. However, they are also cheaper, easier to store, more spacious and more versatile. You can use an oven bag for brining (its flexibility helps it fit in the fridge), or for proving bread, for example.

The only reason I'd ever cook in plastic is if I did not want the mess to clean up. That's the reason for those bags to begin with, to make clean up easier. Aluminum foil will accomplish the same thing, so I use parchment paper to line my pans if I am using real messy ingredients.

Oven bags are useful for cooking in microwaves, as the usual technique for ovens involves foil, which you can't put in a microwave.