Most of the time at reasonable listening levels you are only going to need a few watts 1-10 say to drive a speaker. But that is not why you need a robust amp. Ever hear a sudden orchestra crescendo where the instruments are indistinguishable? Or a cymbal clash that hurts your ear? Or a kick drum that sounds limp? Like most people you probably turn the volume down to get it to sound better or someone yells its too loud. That messed up sound is what a clipping or distorting amp sounds like.
If you have ever watched an amp with metres you can see how they rock upward when loud transients suddenly demand tenfold or more watts . When demand exceeds supply nasty stuff happens like intermodulation and harmonic distortion or fried voicecoils.
How much power you need depends on your speakers and listening levels but it would be a mistake to suggest that the dynamics of any speaker would not be improved by using a more capable amplifier.
Regarding receivers, I'm a big fan of receivers because of their excellent feature set but a receiver does compromise fidelity compared to separates because of the presence of a large transformer in a metal box near circuitboards, wiring and transistors. Great care must be taken to minimize problems such as heat dissipiation and RF interference which is always there to some degree. Separating the amp from the pre-amp virtually eliminates those problems.
So yeah a half watt from a separate amp will usually b e better that from a receiver for many reasons.
