I agree that his crossing is very poor and needs working on which is an understatement to say the least, however we mustn't get carried away with the fact that this is his only job in the team.
In fairness to him he carries the ball out of defence at times very well, turning defence into attack and also does track back a lot with his pace an obvious asset in both of these situations.
I know his crossing and corners are abysmal at times but he also stays out wide which in itself pulls defences out to create gaps to operate.
Let's not be too single minded about why Mr Pellegrini should pick him.