First, I'd like to say thanks to all of you for a well-reasoned and polite discussion.
I only have one camera (7D) and have been debating getting a 5Diii.
That said, it should be noted that the higher spatial resolution of the 7D is only really a benefit in focal-length-limited scenarios. If you are stuck with a specific lens (say the 600 II), unable to get closer to your subject, and unable to add a teleconverter, then the 7D will always be capable of resolving more. In all honesty, that is frequently a contrived scenario, and you more often than not are at least capable of slapping on a 1.4x TC, and in the case of the 1D X (and hopefully soon enough, for the first time, the 5D III as well) adding a 2x TC. For the more skilled, getting closer to your subject is also not a problem, and most professional bird photographers have the sneaky skill to get within feet of jittery subjects, so filling the frame and getting as many pixels on subject is rarely an honest real-world problem when money and time are no object (or at least a secondary concern).
I have to say I don't think the "focal-length-limited" scenario is a contrived scenario at all. For me that seems to be the rule rather than the exception. From personal experience:
1. You're on a cliff on one side of the river and the bear is on a carcass on the other side.
2. An owl in a nest is being mobbed by crows - same deal, can't get closer because you have to drop off the steep slope and wouldn't be able to see anything but trees.
3. You don't want to get closer to the wolf pack
As far as slapping on the teleconverter, I already did that with the 7D.
Now that I have that out of my system, I'm hoping you guys can point out the error in my thinking here:
Given the "focal-length-limited" scenario above, and assuming good light (I know, that usually (or should I say almost always) isn't the case but speaking theoretically here), you want a 16 x 20 print and you crop the FF to the 1.6 dimensions to get equal subject sizes on the final print. I think that gives you 216 pixels per inch for the 18 MP APS-C vs. 150 for the 22 MP FF crop. Doesn't that give the APS "reach" some advantage?