I think RMU should be made as compatible as possible with earler editions. That way, you don't alienate the player base and you have a huge body of MERP and Shadow World (and Spacemaster) modules and adventures to play RMU with. Those who don't plan to use all of RMU might still buy it, then, to use parts of it, like the new spell lists (which have spells at every level) or the piecemeal armor rules.