Why I prefer carrying the full-sized 1911? Well mostly because that's what I like.
Secondly, the 5" 1911s historically have a better reliability track record than the shorter guns. I'm sure a lot of people will say "My 3" has been flawless for 1000 rounds!" or something. That's good. The fact is that the original 5" format is inherently more reliable. I have some Commanders and some 4" Kimbers that shoot great. The 4.25" Commander length is said to be the shortest 1911 format that maintains the reliability of the 5". I will certainly not argue with that, since it has been my experience also. But if you talk to some of the gunsmiths that have worked with the 1911 over the years you will probably hear the same thing: The 5" 1911 is the most reliable of the bunch, with the qualifier that the 4.25" guns follow a very close second, if not tied.
I happen to like the 5" tucked into my waistband. It can actually conceal better than it's short-barreled brothers by the leverage the longer slide imparts on my hip. This will tuck the grip closer to my side. A 3" variant in the same location will not hug my side as closely because the barrel is so short, and can even want to rotate out of the waistband.
I had a beautiful Mitch Rosen IWB for a Kimber 3" that would not want to conceal well from the fact that there was so little gun below the belt. The butt was held away from my body because of this. I called their shop and was told that they suggested nothing shorter than a 4" to be worn IWB. A 3" OWB, however is as concealable as anything, just maybe not quite as reliable with a less-than-perfect grip or an awkward one-handed shot. My Kimber 3" was known to malfunction with a light-handed or "limp-wristed" grip. I don't believe there was anything to be done to the gun to remedy this except hold on to it right. I had reliability and performance work done to it, but this proclivity was still evident. In a bad situation, with a poor grip on the gun with only one chance at making one shot, I would much prefer to have one of my 5" guns in my mitt.
My tendency toward +P defensive loadings also plays a part in my choice of the 5" gun for carry. I have shorter recovery times with a full-sized steel gun than I have with a 3" aluminum framed gun. I think that the slide velocity in a short slided gun with +P loadings can sometimes endanger reliability. Again, this is what I have seen through my experience over the years, and that's really all that I have to go on.
Since the butt of the single stack 1911 will conceal for me regardless of the length, why not have a full-sized grip with an 8 round mag? And if carrying IWB, which is what I prefer for the best concealment, why not tuck a 5" barrel into my holster, especially if I can shoot it a little better, feel that I can rely on it a little more, and actually hide it as well or better than a shorter-barreled gun?
I may or may not be the only guy out there that feels this way, but then again, I'm the only one I'm really concerned with.