No, the model of the world you have can not dominate the DT or, for that matter, do anything at all. There must be a decision theory either explicit or implicit in some action generating algorithm that you are running. Then it is just a matter of how much much effort you wish to spend developing each.

I spoke imprecisely. I meant that the part of the program that generates the model of the world dominates the DT in terms of what action is taken. That is; with a fixed DT you can make it perform any action dependent upon what model you give it. The converse is not true as the model constrains the possible actions.

A Decision Theory doesn't make you naive or impractical. Deciding to look for more information is just a good decision.

I think in terms of code and Types. Most discussions of DTs don't have discussions of feeding back the utilities to the model making section, so I'm assuming a simple type. It might be wrong, but at least I can be precise about what I am talking about. See my reply to Sebastian.