For most people Full Frame makes no sense. But a lot of Very Serious Photo Enthusiasts think they need Full Frame. And CaNiSony marketing wants you to buy FF.
Do you shoot landscape and print really, really HUGE Then you need something like a Nikon D800E. At 13x19 even a pro-pixl-peeper will have a hard time telling what camera was used (M4/3, APS-C or FF).
Sorry, but this is a pretty narrow notion of why people want FF. Here are a number of legit reasons why FF is appealing even to non-pro/non-Very Serious Photo Enthusisasts:
- Better results when cropping images. Bigger sensor means you can extract a portion of an image and still retain better IQ than with a comparable APS-C. I mean, essentially what a APS-C is is a cropped sensor, so cropping an image from a crop sensor is losing that much more detail. In other words, maybe you don't need a 13x19 print, but if you want a 11x14 of a portion of an image (think about cropping a landscape image to portrait orientation), you're better off with FF. Personally, I've had plenty of shots where I crop 1/3 or more of the original image away, and I want the resultant image to look good at large print sizes. That is more likely to happen with FF
- No crop factor for wider angles. This is a big one for anyone who does landscape, architecture, and/or travel photography. An APS-C sensor with a somewhat wide lens (say, 24mm) gives the equivalent of a 39mm lens on a FF body. I dunno about you, but I've had plenty of occasions where I needed a wide angle better than 39mm. For landscape photography, if you're good with setup and software, you can stitch multiple images together. Or you can plow more money into wider-angle glass. But ultimately, the $ difference between a 6D and whatever high-end APS-C body you like is going to be a lot less than the price difference between a good 24mm lens and a 16mm lens (16mm on a crop is roughly 24mm on a FF).
- Related to #1, you don't "lose" reach with a FF. An APS-C sensor crops the amount of the image that the sensor captures, so your 200mm tele on an APS-C isn't really a 320mm--you can get the same effect by simply taking the same shot on a FF and cropping. Or, if you REALLY want that "reach," shell out for an extender. Yes, pixel density is higher on APS-C sensors... but that's not always a good thing, because...
- DLA. The worst FF DLA is f/9.1 (1DM4), and most of the up-to-date models don't have DLA issues until f/10 or so. APS-C sensors, with their smaller pixel sizes, start bumping up against DLA around f/6.9. So if you shoot with smaller apertures on an APS-C body, you're going to start losing detail because of the "advantage" of the smaller pixels. Get a decent long lens with a widest aperture of f/5.6. Put a 1.4x extender on it. If you stop down for whatever reason, on an APS-C body you're going to start losing IQ.
So yeah, I think APS-C has limitations that aren't restricted to a subset of photographers. If you want to get the full benefit of wide-angle glass, if you often crop images and want the crops to retain their quality even at decently large sizes, and/or if you shoot with narrower apertures and want to retain the sharpness and IQ you get with wider apertures, then FF makes a lot of sense. It's not all about the size of the prints, and reducing it to that is just plain wrong.
And for me, I grew up shooting film, manually metering and focusing. A slower/less-precise AF system is an easy trade-off for me. I'd rather have a superior (sized) sensor and compromised AF than a sensor that results in IQ compromises, but a nicer AF.