3 primary reasons I can think of to go with separates:

1) More honest power rating for the amplifiers. Most receivers are rated with only two channels driven at the rated power. Some power amps are rated this way but all of the outlaw amps are "all channels driven" rated so there is more available power.
2) Heat generation aroung sensitive electronics. Since the amp generates most of the heat and it is in close proximity to the electronics in an AVR the life span is likely to suffer somewhat.
3) More electronis noise generated by the power amps again close to the electronics.

Two big reasons to stay with a receiver:

1) Cost. AVRs tend to be less $$$ than separates. Even AVRs with sibling AVPs in the same lineup are not as expensive as the AVP when you factor in buying the amp.
2) Many more choices and most have all the bells and whistles that AVPs sometimes lack.