It's funny how so much of the FF focus ignore the fact that of 90% of the apertures available, there is an equivalent for the FF image on APS-c. It's only wide open. That FF has an advantage... it's always been the flaw in the argument that you need FF for narrow DoF. I can understand why some folks might want that. What I can't understand is that they actually thin the rest of us should.
The good arguments for FF , D800 - and A7r - resolution.
D4s and A7s - low light performance.
Portrait/wedding photographers -the ability to shoot in low light and isolate their subjects using narrow DoF. "The look."
There are reasons not to use FF.
As a rule, narrow DoF is not a reason for using FF, because equivalence says, most of the time, you have every option in APS-c you do in FF. Only wide open does FF have an advantage there. If your the kind of guy trying to maximize resolution with the gear you have, you're not going to be shooting wide open anyway.
I think the coolest thing about a D800 is with it's resolution it lets you shoot wide open at wide open apertures and still get more resolution than you'd get with your APS-c, if you like that kind of thing.
The bad thing is if you have D600 and a K-3, in the long end or macro, the K-3 is going to kick your FF butt.
And it's going to be hard to argue you don't get enough background separation. On images like this the advantage is clearly with APS-c. The most popular thread on the forum with the most likes, is the 300 plus lens club. There's a reason for that. A photographer who knows long lenses and knows what the best bang for his buck is, is going to be shooting APS-c. But the thing is , for me, I couldn't find a narrow DoF portrait on my computer if you asked me to. I look at some of the photos posted by the guys who shoot FF, and you can see exactly why they went that way. They do shoot wide open, and they treasure that 3D look.
To me it's cliche... each to their own.
That being said, if I owned a 31 ltd, 43 ltd and 77ltd, I'd buy a Sony A7r when the price drops.. for the resolution. But those would cost me more than the camera body.
No background separation? pffft we don't need no more stinkin background separation. There are pictures in the post your FF photos thread that don't have as much background separation.
Now lets talk about what having to shoot at a higher f-stop would have done to my shutter speed if i wanted to maintain my DoF on this shot. Equivalence works both ways. The simple fact is, with an FF camera, I could have to reduce my shutter speed so much I got motion blur from the movement of the critter and I wouldn't have had a useful image instead of a keeper.
But hey, the thread is about why you want FF, so refer to the first part of the post.