Even in cases where the center channel carries as much or more signal than the mains, offloading the mains still frees up some power supply capacity for the center channel. It's like the difference between "one channel driven" and "all channels driven" power ratings.
This won't help in cases where there isn't enough "one channel driven" power for the center channel to keep up, but in cases where there is lots of power going into both mains and center (ie when the power supply voltage in the receiver is likely to droop under load) offloading the mains can make a difference.
The point about having bottomless power for music reproduction is also valid.
Finally, stereo amps often have nice analog power meters, which IMO always improve the perceived sound quality
