Chris
The consequences are only visible for people writing their own tools on top of the Arduino library.
Basically you can not compile twi.c with the C++ compiler.
Further on I think it is a matter of taste on whether you prefer self containment and robustness or strict language layering (C code should not know about C++). Probably other arguments exist as well for both options.
Technically it is about the fact that C++ does name mangling and C does not. See Name mangling - Wikipedia for more info on name mangling.
You can easily find the ticked by following the create ticked link and searching for Wire.h
Best regards
Jantje