If Donald Rumsfeld were here he would say to you:
There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.
The point of this book, obviously, is to turn people's unknown unknowns into known unknowns. Not for them to become competent programmers of 7 languages in 7 weeks.
I had a class on programming languages in college. It was one semester and went into very little depth of each language and paradigm covered. That said, I didn't know that half of the stuff that we were taught about existed before that class, and it spurred me to learn more and to learn at a greater depth.
I doubt that there's going to be any illusion about what this book is. Surveys are vital for new learners to help them build the cognitive maps of the knowledge space they need to be able to understand what they don't know. Maybe you don't talk to a lot of working programmers, but there are a frighteningly large number of people who just don't know anything about programming language syntax and paradigms not descended from Algol-68
There is a lot of value in knowing what is going on in your field.
It would be nice to touch briefly on 7 other, very different, languages (especially Scala and Ruby) and see what they taste like.
So, yeh; I reckon this would be a good book for an already proficient programmer to pick up the beginnings of a few new languages.