The hard line approach (AVR = bad vs. sperates = good), is not a judicious stance. The capability of power amps (within AVRs or not) need to be dictated by the choice of speaker (efficiency and impedance profile) and size of the listening space (10'x10' room or big theater).
The economies of scale make AVRs the best bang for buck on the processing side. The Marantz pre-pro with nearly identical internals to their top AVRs are about the same price. It would seem the latter, with the included amps as a freebie (one more than sufficient for a majority of home AV applications) leaves only one justifiable argument for the pre-pro. (The argument being, a buyer already has satisfactory power amps.)
The permanence of a quality dedicated amp, matched appropriately to speaker/listener demands, is undeniable.
I'm a firm believer that offloading the LCR to a dedicated amp (if needed) and using the AVR's internal amps for remaining speakers, is the best approach.