This is one of the most valid points made towards power amps. Power supply has changed very, very little over the years as compared to other features in the AV world. Sure thre are digital amps now a days, but you can still power any speaker with the same amp you bought in the 70's as well. So when it comes time to upgrading, why repurchase the amps inside the reciever? If as you said you purchase a nice power amp (or two, three, etc...) and either a fairly economical reciever (read - not so beefy amp section) or a pre-pro, then when your reciever/pre-pro becomes outdated, as we in this hobby know all too well happens in the blink of an eye, then you don't have to make such a large investment to catch back up.
This was actually one of my main motivations towards my Emotiva purchase. My Denon was sadly out of date, and only falling further behind. When I purchased it I spent almost $2000 dollars so that I could have a pretty hefty amp section included. I didn't want to have to spend another $2000 bucks on another Denon just to get an extra 10 - 20 watts per channel when I could buy an amp that pushes 400 watts per channel for a quarter of that price and then look for either a much more reasonable reciever, or possibly a pre-pro.
I'm very happy with my choice.
