Why is skeuomorphism so bad?

I've been hearing a lot of talk about this recently, and it's sort of confusing. It's usually been in reference to Apple and their software. For example, the Notes app is colored like a real yellow notepad is, or the Calendar app on OS X ML is designed like a leather planner. The Reminders app is designed like a pocket book. It's really just skinned all over the place. Then there's Passbook, Game Center, and Contacts on OS X.

And I can't see why that is a bad thing. It doesn't inhibit functionality, all it does it look different. Can someone explain why this trend to design apps as if they were a real thing is bad?