Some cameras let you swap the function of the half pressed shutter button and the AE lock back button, in my T2i, the setting 1 at the CFn9 menu lets you do that. Why would I want to use the AElock button to lock the focus? and vice-versa
It is a matter of personal taste and each branch has their own defaults and controls which give slightly different behavior. If you are after a particular behavior, you must check what each model offers.
A good number of photographers, particularly journalists and street photographers, use back-button focusing. This lets them lock focus once and then fire a series of shots without AF delay in between. The difficulty with this is that they must be very careful to keep their distance from the subject constant. In some cases, like an interview or press event, this is usually easy since the parties are standing or seated in a relatively fixed and crowded position. In other words, they will not move that much.