At the moment, the combined userbase of AOS, MOS and AROS is small - having a set of guidelines for portablility should mean that we can pool our apps. Unfortunately I have to agree that the three will diverge - AOS merging with the DE, MOS using the QBox, and AROS doing someting else once perfect AOS compatibility has been achieved. But hopefully by that point each will have enough users to proceed their own way. In the meantime, Open Amiga should benifit everyone, and not affect coders badly.
I don't think that we should aim for complete POSIX complience - I don't know much about it, but if everyone coded for POSIX wouldn't we just be better off ditching AOS/MOS/AROS :-?
