pYro_65:
Problems may arise when ( code bloat / errors ) two sketch files use the same library files.
I think if only one sketch file actually does the #include xxx.cpp inlining, and the other doesn't, it should be OK. The second one would still just see an empty library source file, so no bloat (it still needs to do a #include <xxx.h>, of course, to pick up the header file definitions).
Or if the library includes a file that also uses the stuff defined in the cpp, it could have undefined reference errors.
Yeah, I think there will be problems in the general multi-file case. But nothing a bit of judicious guard #defines and structuring shouldn't be able to work around, I suspect.