He may score some beautiful goals but he's consistently average at his main job, which is as a CM.
For example, Daniele Conti of Cagliari also scores some beautiful goals but that doesn't mean that he isn't average.
Give me one link of Conti's beautiful goal you talked about.
He had done some good job, just look at that Bologna video and you can see he had done his job well. He make interceptions, some good passes, and heck even dribbled some players a couple of times. My following argument is a little bit lame, I know, but it stands: when a CM can dribble past some players more than the wingers (including Krasic), can he labelled as average? My point is that he has the technique that no average players have, it's evident in those beautiful goals but it also shows in other circumstances.
He has good defensive skills but Vidal is better at that and Vidal is much stronger, I can give you that.
Piazenza is average for instance (needs more playing time though to make a fairer judgement), and no way he can be compared to Marchisio.