You can ask this question of 1,000 people and get 1,000 opinions. All that matters is the decision of the crewmember dealing with the issue.

There is no particularly good reason to deny this request, but most carriers (we'll presume you mean EK as you have apparently posted the same question on the EK board) require a passenger to travel in their ticketed cabin in their assigned seat. Thus, if a crewmember wants to be a stickler, that crewmember could certainly require that you sit in your ticketed cabin and seat and there is little you can do to argue about it.

Others who failed to read your post are asking about the situation where people wander between cabins.