Don't forget. LibreOffice will probably have a much easier time of it integrating Symphony than will OOo!
MOST of the effort in the first few releases of LO has been directed towards removing dead code, refactoring redundant code, translating the comments into English, and a whole host of *programmer* *friendly* changes.
OTOH, OOo was (still is?) a pretty ugly spaghetti monster. Okay, they could probably take and use a whole host of patches that are basically just "delete this useless code, delete that useless code", but it's probably almost more effort to review it than just redo it from scratch.
That's a good reason why LO is likely to start leaving OOo in the dust - it's far more developer-friendly, and once you get a commanding lead, it's hard for the other people to catch up ...