Zen1 is right in that in a diamond midfield, its very good to have another good attacking and creative option to balance things up (like a Seedorf supporting Kaka etc), and help out Diego. Our midfield is very strong, but its mainly defensively (even if Melo is decent offensively, its not like those other players), it can get bit bogged down offensively with that midfield if Diego is off, and we lack good fullbacks.
So yeah, another threat balancing things up would be good, another example being Diego's partner in crime last year, Özil, that kinda player can be real good.