Why? Well, eventually, there will be a bunch of RB3 DLC that will support harmonies, and it'd be nice for a family friendly game to support harmonies as well. Not to mention other RB engine enhancements coming in RB3.
I don't see it coming out till next year if at all though. But I can fully see new kid friendly versions of the game coming out every few years, particularly if they keep the every 2 year schedule. Next year, we get LRB2, 2012 we get RB4, 2013 we get LRB3, etc. Not flooding the market, and providing folks with things they want.