This is the question that faces our raid team, two or three nights of raiding a week. It wouldn't be much of a question if this didn't mean adding more people, it does. There is a lot of different ways to look at this but I'll look at it from the clearest perspective I have. How will it effect me, pros and cons.
Pro, we will be able to work on heroic toc and finishing Ulduar, which I very much want to do.
Con, the "new" night will be when I cannot play, on Tuesday. This means the first wing or two of ICC will be closed to me, with it's 5 or 6 drops and 14 or so badges a week.
Pro, not having to worry about making every raid. Right now if I can't make a raid it might not happen, with more people it's less likely.
To me these are the big ones. There are other issues but most of them aren't my concern. As much as I want to see us finish other stuff I really don't want to get back to the way it was for tier 9. Regular toc and ony were cleared on on Tuesdays, I didn't raid on Tuesdays. My gear sucked and so did my dps and healing. We worked on heroic toc on Thursdays where we downed one or two bosses or worked on Ulduar achievements. Without doing the easy content I was undergeared for the hard content.
Right now my my healing gear and dps gear is moving along nicely. I can heal any encounter or dps and perform well. If our schedule changes I will fall behind, no way around it. I'm up for whatever the guys want and think is best for us. Glad I don't have to figure this out. That's all.