So I ask you FF shooters: what is it that you can do nowadays with you FF that you would no longer be able to do if you downgraded to crop?
I'd miss the thinner DoF you can achieve with FF, for the same framing. To get the FF-equivalent of f/1.2 on APS-C would require an f/0.75 lens...last time I checked, there weren't any in a Canon EF mount.
If your long shots are at the long end of your current zoom lens and you shoot from a distance, you will miss the reach.
Well, we've had this discussion before.
The reach only matters if your output demands it. A FF image cropped to the same FoV of an APS-C sensor will have essentially the same IQ - the only thing you're really giving up is megapixels. So, if 7-8 MP is sufficient (which it is for web, slideshows, and prints up to ~12x18" - and I suspect that covers most people's needs), then a cropped FF shot will do just as well as an uncropped APS-C shot.
Does it possibly make sense to stick with a crop sensor for telephoto/sports needs and a full frame with a "normal" lens for general photography?
From a sensor standpoint, not to me (again with the caveat above regarding need for high MP output). Especially if you're not focal length limited. One of the keys for sports is a high shutter speed to stop action, and the much better high ISO performance of a FF sensor means you can push the ISO higher to get a shutter speed that stops the action. With the sensor in the 60D (I have a 7D), I really prefer to keep the ISO at 1600 or lower. With the 5DII, I had no problem shooting at ISO 3200. With the 1D X, I routinely use ISO 6400, and I'm fine with ISO 12800. That's 3 stops
better than I prefer on the 7D (although I can tolerate ISO 3200 on the 7D, so call it 2 stops to be conservative...but still that's the difference between a blurry 1/250 s and a crisp 1/1000 s).
Of course, the sensor is only part of the story for sports/action. The AF system is the other big part. For fast action, I'd take the 7D and live with the noise vs. the 5DII/6D and lower noise. The 7D's tracking capabilities are far superior to the 5DII, and will be similarly superior to the 6D. But I'd take the 5DIII over the 7D in a heartbeat for sports/action - FF for higher ISO and even better AF more than makes up for the loss of 2 fps.
I can tell you that after getting the 1D X, my 7D has just gathered dust. As I stated above, IMO a FF image cropped to the same FoV of an APS-C sensor will have essentially the same IQ. I tested that semi-formally with the 5DII vs. 7D (with a static test scene) and proved it to my satisfaction. I've actually decided to test my statement above with the 1D X vs. the 7D, with a 600/4 lens, comparing the 1D X and 1.4xIII vs. the 7D (as approximately equivalent focal lengths), and also at the longest AF-capable focal length (1200mm f/8 on the 1D X, 1344mm equivalent f/5.6 on the 7D). This will be a 'formal' test with an ISO 12233-based chart, and a static 'real world' scene. If the 1D X cropped gives equal or better IQ vs. the 7D, I'll need to decide if I sell the 7D or keep it solely as a backup camera.