I personally think that they look so much better at the bottom of the UI like mine:
http://www.wowinterface.com/download...o.php?id=21221
However I can understand that having them more in the center would be easier to look at especially when a boss hits you for a ton of damage, you are quickly notified.
I like how my UI is arranged because I have the spells and unit frames so close together that I can see everything in the same place. I try to limit myself on how much I need to look around my screen to see information. If I had the unit frames in the center then I think I would want the action bars there too which would look horrible. Might try having them in the center one of these days though to try it out.