I can't speak for anybody else, but IMHO, variety is the spice of life. I'd hate to see either OS4, MorphOS or AROS fall by the wayside.
I've always wished I had the time and sufficiently in-depth knowledge of each platform to build a common runtime for deploying applications across them all without having to sacrifice too much in striving for the least common denominator. I realise, for example, that MUI and Zune work very differently to Reaction but in all honesty, there's no implementation difference between them (or indeed almost any GUI system) so great that the appropriate application of design patterns couldn't abstract it.