My reasoning for Propecia over generics is that it's the only one I can really trust to be what it says it is, since it's officially produced and sold as a proper product. With generics, although I am sure many are fine, you don't get that guarantee.
Perhaps in years to come the generics - like with paracetamol and most other easily available drugs - will be just as good (perhaps even Propecia will depreciate in value).