thesnark: I think the past has clearly shown that neither RO Ltd. nor Castle really have the development resources to take RISC OS forward. And both have no intention to make many of their developments available to the other party (and therefore to the whole RISC OS userbase).
I think it is highly unlikely that both branches of RISC OS will merge. Therefore, I see the RISC OS Open source soon becoming the de facto standard for all RISC OS users, since this will be the RISC OS branch which allows OS development for all users. In the long run, it will help to prevent future incompatibilities and future wheel reinvention.
I believe this is the only way forward for RISC OS. The only way to bridge the gap between the currently existing branches. The only way to attract new users.