Forget that whole "1ME is a lot of money", line of thought here. When you play for Juventus, a salary of 1ME is definitely not a lot of money.
The guy has started almost every game he's been fit for over the past couple of seasons. The management has labelled him a player this team's future will be built around, and he makes relative peanuts. 5th lowest on the team right now, and he was 3rd lowest on the team before Traore and Rinaudo were signed on loan.
We're a big club, with a big payroll, the least we can do is pay a guy like Marchisio 2ME. It's a fucking investment for the future. He's previously played better for us than he's playing right now, and in the future, he'll be better for us once again. A raise to 2ME would surely make the man feel considerably better about his standing in the club.
Everyone likes to feel appreciated, and if I were in this guys shoes, I would be questioning how much those in control of the club appreciated my presence on this team.
When people are unsure of their standing and their future, they often don't perform up to their capabilities. You can question whether this is wrong, or right, but it's certainly something that is very frequently obvious in the world of professional sports (and in the real world too, there's just less blatant coverage about it).