Migrating GCC to C++ as implementation language:
C++ is a standardized, well known, popular language.
C++ is nearly a superset of C90 used in GCC.
The C subset of C++ is just as efficient as C.
C++ supports cleaner code in several significant cases.
C++ makes it easier to write and enforce cleaner interfaces.
C++ never requires uglier code.
C++ is not a panacea but it is an improvement.