What benefits are there to programming an MSP430 in C/C++ over Energia?
I think that the main benefits on a full-fledged environnement (as Code composer) over Energia:

1) Able to program the whole family of MSP430 microcontrollers, and not only a few selected microcontrollers as in Energia.

2) Able to debug the applications by setting breakpoints, watching variables, step-by-step, etc.

3) Able to use a much more capable library (MSP430 Driver Library) in comparison with a limited arduino-clone Library. For example, I missed in "standard Energia" the possibility of setting a timer that triggers an interrupt; you might of course install a third party library for doing so or playing directly with registers (though you might interfere with Energia code, which might use this peripheral for some other purpose). 

4) In contrast, using MSP430 Driver Library you can program every existing peripheral on the microcontroller; and you know what peripherals are being used (because you are programming all of them instead of relying on hidden code as in Energia).

That said Energia is great for quick prototype. Both environments are worth to learn.