Does Diablo contain some application/library/system setting/something from Nokia or Community that Fremantle doesn't have?

I can put it another way: If someday Fremantle reaaly works on N810/N800 what are the reasons not to update from Maemo4 to Maemo5?

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In my utopia, Maemo5 is *up to date* and N8x0, N900 and N9 all uses it with every application from Maemo4/5/6.
(With device limitations: If application needs accelerated 3D, it can be used only if device supports it. Or if application needs RFC/GSM and so on.)