Picking up Andrew Hodgkinson's points. I do think this is a positive step, and I didn't expect it to happen. Congratulations to those who have achieved this.
Fragmentation is a possibility, and there could be incompatible changes made, who will decide between them? However I suspect there won't be much of a problem in reality.
As a grumpy developer I hate different versions of RISC OS working in different ways - it is no fun at all having to do work for the sake of 10 people who have version X of RISC OS. RISC OS might have had a new generation of scanner drivers if I hadn't used up so much time with the Simtec/Castle USB API episode.
Yes on reflection we have had a some things I predicted before with Select/Adjust. Two things might be different. Firstly Select was effectively a complete version of RISC OS. You didn't find someone using random components of Select on RISC OS 3.7 or the Iyonix. Secondly Select was a paid for upgrade which limited the numbers using it. There is an assumption there, open source == free software?