The only reason to get a seperate power amp in my opinion is for power. Most receivers are going to put out a nice safe amount of power perfectly suitable for most listeners needs. But I don't think most of the people here are "most listeners". If you have a pair of M80's for example which are 4ohm speakers and designed for high volume levels , they do sound better at higher volume levels with more power, especially the bass. I moved from about 200 watts to 600 watts and the biggest improvement was in the bass department. They will now pound out the bass much louder. I have a sub, but for 2 channel music I like to go full range with the M80's.
I don't see the point of buying a 2 channel amp that puts out 150 watts, when a Denon, HK, Yamaha all put out just as much with all the processing thrown in too, into 5 channels, many times for about the same money.
M80ti's, VP150, QS8's