Ok, well my copy definitely doesn't have this issue, and the "issues" from earlier had everything to do with the lighting being poor. So, instead of a bookshelf on the side of the room, I set up a quick & dirty 'photo shoot' directly under the fan, which is plenty of light. Center point full size link
The setup was five nice, contrasty crayons on a scarf. I took five photos each using the center point, above-center point and below-center point. This lens has not been adjusted for my 70D at all; I just got it and thankfully it doesn't seem to need it.
Test results follow. There will be links to the full size files on my site. They are quite large (~9000px wide, ~2.5MB/each), but if you feel like seeing them, they're there. Everything is SOOC JPEG. ISO set to 200, aperture f/1.8, auto shutter speed (Av mode), on a tripod. Just cropped to the crayons (~50% or so crop) and joined together. The AF system was set to one shot, single-point and it was re-focused each shot.
70D, Center Point, EF 50mm f/1.8
.70D, Above-Center Point, EF 50mm f/1.8Above-center point full size link
.70D, Below-Center Point, EF 50mm f/1.8Below-center point full size link
Remember, this is far from ideal lighting. A single 100-quivalent daylight temp CFL in a ceiling fan about five feet above the 'scene'. I didn't even have to go outside and get real light. For my part -and my 70D copy-, I consider the matter closed. There isn't a thing wrong with the center point. Certainly there may be defective models out there, but I don't think it has to do with any sort of overall model defect on Canon's part. If it were, many
more people would be doing this kind of testing and failing.
If you're having trouble with the AF system, but can get it to pass this kind of test as easily as I did, that just means you have to get to know the AF system better and use its behavior to your advantage. My copy, too, seems to default to the nearest object in the field of view quite often when using multiple points, so I choose my own AF point instead of trying to let it do so for me. That "issue" has nothing to do with the one spoken about in this thread.
EDIT - Woohoo, this is my 100th post.