The more I read, it seems that crop bodies have a singular advantage over full frame and that is the increase in focal length.
If you are 'focal length limited', i.e. need a longer lens, with FF then yes, crops have an advantage. If you're not then they don't.
The main advantage of full frame (to me) is being able to get a sharper drop off from the in focus plane to the out of focus plane. How is this achieved? To get the same framing on FF as on my crop I have to use a longer focal length... and it is the longer focal length that gives you the sharper drop off. In addition the depth of field is shallower in the first place.
Pretty much everything else can be regarded as minor or for pixel peepers only. If you're not making very large prints you can ignore lots of info :-)
(one other thing to add is that the FF bodies have better ergonomics than the Rebels and a larger, brighter, viewfinder. My 550D is only used when I'll be cropping (my FF is only 12MP) or am focal length limited, such is my preference for the ergonomics and viewfinder of the 5D)