What does AI-Focus do?

AI servo is not ideal for a stationary subject as it constantly tries to refocus, this can lead to a slightly out of focus image if you hit the button at the wrong moment, this doesn't matter with a moving target as you have to constantly refocus anyway, that's the point. So for a stationary subject use One Shot Focus, you will get much better shots much more reliably. For a moving constantly moving subject use AI Servo as it allows you to track and constantly refocus on your subject.
AI Focus is for situations say like bird photography. A perched bird is best shot with One Shot Focus, however if that bird takes off it is best shot with AI Servo, AI Focus is supposed to handle this change where a subject can go from stationary to moving without warning.
The reason why you cant always use AI servo for stationary is if you commonly focus withthe release pressed half way and recompose ( which is typically how i shoot stationary subjects.) The focus will adjust when you dont want it to.

As I said in my original post, that you quoted, even if you don't recompose, AI Servo constantly refocuses anyway. With the amount of AF points on modern SLRs there should be little need to recompose. And of course, the main reason not to use AI Servo when shooting a stationary object is that it simply isn't designed for that purpose.

when you use AI focus, the little lights light up in the viewfinder, if you have it set to auto focus. With AI servo, it will still pick the points of focus automatically, but the lights won't light up so you don't know where its currently focusing. A slight downside I've found out.

Though unless you have a 7D or something higher, I'd stay away from auto focusing in most cases where it can be avoided. It's simply terrible.

I have a 7D and you can select the primary autofocus point for AI Servo. It will always focus on this primary point, but will also try to focus more points as well.
 

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