I don't why he always misses easy shots that are almost clear cut opportunities. But one good thing about Benzema is that he is a creative forward. He somewhat posses qualities of a midfielder. I think it's best for him to play behind the first striker. He is good in linking up.
I don't know if we can say he is that overrated as his is a very technically gifted player and can provide for his team in a variety of ways other than scoring. On the other hand he is a striker and STs are to score goals first and foremost. And on the other side of that other hand (what is that 3 sides now?) he is a great compliment to Ronaldo who obviously takes good care of that scoring department. Karim is a great player and you don't suit up in a blanco kit unless you are just that.