Finishing is different from scoring. Some players score a lot of goals because they get a lot of chances or create a lot of chances themselves, finishing is about being clinical when the opportunity arises, think of someone like Michael Owen. When you think of what chances Marchisio actually gets to score and the difficulty in the goals that he does score I'd say he was one of the better finishers at the club, especially for midfield, he's wily in that respect. Again, this doesn't take into account his relatively poor long shot.
I too would like to see much more from him, he's underachieving. To answer the question I'd say he was 5/10 on form, 6/10 overall as a player, and with the potential to be 7.5/10, with some games as 8/8.5.