Given our limited numbers I see little point in conquering any of the Ork worlds. It would be a massive investment for room and resources we don't currently need. For the moment simply destroying their fleets, and possibly bombarding whatever industrial complexes they have into craters, removes them as an immediate threat.
It was a direct recommendation from the Security Council staffed with military geniuses, so I take the idea very seriously. Durin rarely has the NPCs come out and tell us that a strategy is a good idea like that.
As far as I understand it, the idea is not so much that we need the world as it is that the Orks would be overwhelmingly attracted to the idea of reconquering it and more inclined to leave the rest of the Trust alone so they can attack there. It would be a lightning rod for Ork invasions, taking strike after strike so all the other systems are safer.