Dynamic language developers who are concerned about performance end up writing pieces of their applications in C++. So if you’re going to write C++ anyway, why not write your entire application in C++?
Library writers develop in C++ so that their users won’t have to. That makes a lot of sense. But if you’re developing your own application, not a library, maybe it would be better to write everything in C++ instead of wrapping C++ in something else.
A few years ago an immediate objection would be that C++ is hard to use. But with the advent of C++ 11, that’s not as true as it once was. C++ has gained many of the conveniences traditionally associated with other languages. (Update: for more on this theme, see Not Your Father’s C++.)
Dynamic languages are designed for programmer productivity at the expense of efficiency. If you can afford that trade-off, and quite often you can, then don’t worry about C++. But if you’re using a dynamic language and C++, maybe it would be easier to just stick to C++. Of course the decision depends on many factors — how much of the application needs to be efficient, the size and skill of your team, etc. — but I suspect more projects will decide to do everything in C++ because of its new features.
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 “Productivity” implicitly assumes productivity at a certain set of tasks. If your tasks fall into the set of things a language was designed to support, great. But a “productivity” language may not improve your productivity if you don’t meet the language’s intended profile.