I think there are genuine advantages to Select over RO5, although they may not be what you're looking for. The things that stood out for me (when I did subscribe in the past) were icon cut and paste, improved filer (e.g. thumbnailing), better networking (e.g. instant effect configuration changes) and sprites with proper transparency (great for icons).
However, the biggest benefit that Select is likely to bring is a unified OS across the full range of RISC OS machines, meaning that the "under-the-hood" advantages of Select can be harnessed by programmers properly.
Moreover, I don't think it's in Castle's interest to be worrying about maintaining the user-facing parts of RISC OS, like cut and paste in writable icons. In my view, Castle are much better placed to deal with hardware and the more fundamental improvements (HAL, USB support etc.).
Personally, my only worry is that some of the improvements found in RISC OS 5 (such as good USB and graphics card support) will be lost in Select. This would really be a bad thing and I hope ROL can manage to provide the best from both versions. The sooner ROL get started on Iyonix Select, the sooner this issue can be dealt with.
This is my personal reasoning for wanting Select on the Iyonix: it's the only way we're going to end up with a unified OS. Let's hope this is ultimately what Phil's pledge brings about.