After nearly 10 years using Av mode almost exclusively, I've now switched to TAv mode with my new K5. Here's why:
I used Av mode in the past because it was the most sensible for film. My ISO was set with film, and shutter speed was the setting that I was most happy being set for me. I wanted control over aperture. Once I moved to digital, I still set the ISO because it was pretty limited on my *ist DS (what a terrible name...). Going forward to the K10D, and then K7, my ISO was more flexible, but still pretty limited. I'd suffer with a slow shutter speed and use the SR to it's fullest because the high ISO was poor.
Now, going up to 3200 ISO is pretty damn good in decent lighting. I can now lock my camera at f/4 and 1/125 and let the ISO float. It's fantastic. It's allowed me to get shots like this, because I didn't have to worry about a slow shutter speed, nor terrible high ISO!
For the record, this was in TAv, f/6.7, 1/1500 and ISO 2200.