Prefixing all compiled rbc files with a . would help hide them from the (power-)users and significantly de-clutter directories/home-dirs. Prefixing additional cache file names with a . is a common strategy used by many text-editors and file processors.
Python has pyc files in the same vein and they're provide to be no big deal. There is a flag to disable .rbc file generate which can be used (-Xcompiler.no_rbc) if the user is really sick of them. Prefixing them with a . to hide them in certain contexts introduces other issues, thusly, I'd prefer to leave them as visible. If you have an alternate proposal, please let us know.