We will never use Valencia on the left because he'd be useless there, he doesn't have a left foot and he's not very good at cutting inside (though he's done it too seldom to judge). Nani, on the other hand, is very good at cutting inside, and fast enough to create space to produce better delivery there, he won't cross it as good as he does on the right but it'll still be decent and I reckon goals he could produce from in there will more than make up for it - like Robben at Bayern, he doesn't possess the greatest right foot in the game but he does well on the right hand side.