Some do. Most don't. I agree with Meiwan and go the 2 channel integrated amp route.
First, your not really paying for extra channels in an AVR as most of the R&D and parts costs go into features like video processing, room correction, etc. Not amplification, either preamp or power amp. The built-in amps are generally low power with small, off the shelf power supplies. At least the mainstream AVR's definitely are. There are a few upper eschelon manufacturers of AVR's that will pay attention to quality amp sections but at these price points, you can get pretty high end integrated amps instead.
Since you don't need any of this stuff along with the extra low powered channels, look at a quality 2 channel integrated amp. Plus, many are now starting to come with built-in DAC's. The critically reviewed Harman Kardon even has bass management and sub outputs. If you want 2 channel, then buy quality 2 channel equipment that is designed for it.
