Ok, I've just read this and still dont get why someone would use AV or TV for any reason other than laziness. And I certainly don't get why using manual and the camera's meter is the same as shooting Auto!
I'm not eloquent or experienced enough to put forward an argument, but here is my story...
When I started this photography thing 6 months ago I had a quick read of the manual, and starting shooting AV (aperture priority). It seemed to offer the right sort of creative control to me. But, everything came out crap! Bar the odd shot. From posting on here, I found out that it is the camera's meter. It is easily fooled. So I did a little more reading and came up with 2 options. Either dial in some -ve/+ve exposure comp or learn how to shoot manual. I chose the latter, and couldn't believe how easy it was. Set camera on spot (or the 400d's equivalent, which is a big spot

). If the bit you're metering is dark, make sure the meter reads under, if the bit you're metering is bright, make sure it reads over. Shoot. Look at the histogram and if you think you can push the histogram a little more to the right, do so. Or if you've completely blown it, stick the shutterspeed up a few notches.
Surely this is the same as AV. Except rather than under/over reading on the meter, you'd dial in the exposure comp based on what whether the bit you're metering is dark/bright.
If shutterspeed is more important than DoF, then I set shutterspeed first, and apply the same process adjusting the aperture to obtain correct exposure.
So, why would AV or TV be beneficial? I must be missing something. :shrug: