Is the title of this thread "center AF vs 9-point AF" or is it "center AF point vs off-center AF point"?
The title and the question aren't necessarily the same, or could at the least be misinterpreted. Especially since you cannot actually use all 9 AF Points at once. As already pointed out, activating all 9 point vs selecting a single AF point allows the camera to make the choice for you, which can and often will choose the one you don't want.
the T2i has 9-point AF, however, i keep mine on center point AF always. this way, i can pick what i want to focus on, and recompose as necessary.
am i giving anything up by using center point AF 100% of the time?
Yes and No; It depends on the situation! When shooting stationary subjects/objects, using the center AF Point to acquire focus then recomposing the shot works fine and is probably the most accurate focus point for most lenses. But then you would probably be using a tripod and could be focusing manually.
On the other hand, if you are not shooting stationary subjects/objects where you are not able to focus then recompose the shot, E.g. See above photo, then one of the other AF points might be advantageous for you.
There will be a number of situations when using one of the other AF Points is a better choice. E.g. Moving subjects and/or a very narrow DOF.
Other advantages to using one of the other AF points, more specifically the six surrounding the center point, is that you can frame/compose your shot on one of those AF points utilizing the rule of thirds; rather than centering the subject on the center AF point then cropping the picture in PP. You might not want to crop that much to create a properly composed picture and using one of the other AF points can save you the trouble.
Most of my photography is outdoor stuff such as Birds which are constantly on the move. I have tried turning on all 9 AF Points. It's a big PIA, I wouldn't recommend it.