Other alternatives (like RiscOS) will be used by people buying the Pandora specifically for that OS (or maybe multibooting if possible) and would probably never have bought a Pandora otherwise, therefore they are not fragmenting the standard market, they are just a niche set of customers boosting sales.
Personally, I'll be sticking to Angstrom for quite a while but if the Amiga oriented OS's (AROS or Anubis) get useful I'll be using them too, probably booing from SD so that I don't affect my core, supported, Angstrom distro. Since Anubis is planned to be Linux based I'll be looking to help out getting it fully running on the Pandora hardware and maybe adding in some PND support, but I see this as expanding the Pandora into new markets, not fragmenting the existing one.