I think this is is very encouraging. It means at the very least that developers can fix those anoying little bugs or add a missing bit of functionality and easily and have them merged into the distributed releases.
For larger projects, there is a load of potential (for example filecore/fileswitch
64 bit support for hard disc MP3 players). The problem here is that large projects such as these need a lot of expertise and time which may not be available.
I don't think it will cause a proliferation of incompatible versions of the OS, not least because there are insufficient people with the time and skills to do this. For the forseable future, I think the majority of users will stick to official releases as a guarentee of quality. Similarly, I would expect most developers to develop for the official releases and not rely on custom enhancements.
I hope ROOL provide CVS or SVN repositories for public projects, aiding and encouraging colaboration and guidelines on new APIs and the like to avoid incompatibilities.
RISC OS Ltd. could sulk and refuse to accept the new regime, or they may be able to take advantage of it. They may be able to add these much talked about APIs to RISC OS 5 to allow more of their extensions to run and sell to Iyonix users. It is up to them to take advantage of the possibilities.