In Av or Tv, what happens when you want to meter one area, but focus on another area?
Usually the answer is to set the camera up so that the exposure lock is on a separate button than the focus lock.
Bingo! There's my answer. I don't have the capability to separate the exposure and focus lock. They are both on the half shutterbutton push. Thus, I usually am in manual mode rather than Av.
Example, if I am taking a shot in the backyard of the kids playing with the dog and want to focus on one of the kids, it will also meter said child and on a bright sunny day, my light colored yellow Lab will be completely blown out.
Of course there's always exp. compensation, but I normally run -1.0 anyways and that doesn't leave me much room to move it if I'm in Av. Besides, in my opinion, using Av or Tv and exp. compensation is no different than using manual mode as you are overriding what the camera wants to set anyways. So, why not just use manual anyways instead of messing around with exp. comp?