Blizzard Senior Community Rep Jonathan "Zarhym" Brown engaged in an interesting conversation with a Twitter user yesterday, regarding making Siege of Orgrimmar cross-realm in normal mode as well as in Flex and LFR. You can see the whole conversation
after the break as an image, but the jist of it is that the OP wanted it cross-realm, and Blizzard can't do it. The framework is apparently not in place, despite it being there for flex and LFR, and they feel the failure rate would be too high. Not to mention that any additional development work that was done on Mists
would affect the delivery of Warlords
Cross-realm normal mode raids are something that has been asked for for a long time, since LFR appeared almost. In the twilight days of Mists, it might be a good way for raiders to spend a bit more time on the content and might elongate the life of Siege. But is it really worth potentially delaying the new expansion? Given that the delay between now and the new expansion is the whole reason why we'd need it in the first place, it doesn't seem that way.