I think I did make the distinction in my first post about 'games when we are expected to have more possession and make things happen'. Naismith can be decent in these games cos of his willingness to work, and we are generally further back in our play. But when we are in possession much further up the pitch, he often just gets pushed off the ball, or loses it easily. I know Lukaku doesn't posses the greatest of touches either, but he is strong enough to at least occupy defenders and provide a threat, which often leads to better opportunities for the players around him. He is a far greater threat in the box as well.
I think I already made the point about when both play, we lose one of the wide areas, or the Barkley influence in the final third.