Originally posted by Ratcheteer and don't forget subject isolation, having a sharp background, or funny bokeh aberration that pixel shift causes can distract from the main subject if its in the foreground which is the number one reason why people want full frame in the first place (less pissing around in post)
Most people who use this as reason to justify full frame really have no idea what the depth of field for a F1.4 50mm or 85mm lens looks like on a full frame camera.
The depth of field is SO NARROW that it is really only usable in VERY and LIMITED specific situations. F2.8 on an APS-C camera is MORE THAN sufficient for isolating a bride for a very close head shot bridal portrait, for example. Her portrait would look ridiculous if her nose were in focus and her eyes weren't or her eyes were in focus but her nose wasn't.
As someone else said, I think Full Frame is great. But only very marginally better than APS-c and the gap is closing.
In practice I have been shooting APS-c weddings since the Canon 20D and NEVER had the reason or desire to shoot FF weddings. The only thing I wanted FF for was resolution and rendering in landscapes, and APS-c has killed that need/desire.
I will probably buy a Pentax FF if it's any good.