2 primary reasons:
1) It's for wife and two infants.
2) My old Panasonic Plasma has only 1 component video input.
Rather than to switch through inputs on TV AND A/V selection on the receiver, I have my AVR perform the up-video conversion. Therefore, my wife and kids have to hit only one button to turn on every thing. However, they sometimes turn the remote 180 degree b/c they turn to talk. Thus, not all components get turn on. This is when they know what to do to fix the problem. I try to keep things simple for them as much as possible. Sure, you lose some signals, but to my wife and kids, they couldn't tell the marginal difference. High-end interconnect cables with power conditioner help.