An Altitude Hold function is required for RVSM airspace.
True enough, however one is not required to use it, it is just recommended. One is still allowed to hand fly in RVSM if one is confident enough in his skills to do it within RVSM limits. Of course it becomes kinda boring after some time and that is where an autopilot has its use. But there is nothing preventing us to fly a SID or STAR to/from cruise level manually without a FD. Yes. even if it is PRNAV, it is not really all that difficult.