CrossRoads:
Another advantage might be interface speed.
MAX7219/7221 can receive data at 10 MHz. I use mine at default SPI settings which I think is 4 MHz.
If your main processor was really tied up and you wanted to send stuff out at 8 MHz to get it out of the way quick. The master can do that, I think testing has has shown a 168/328 cannot receive that fast tho due to the slave having to sample (the CS pin?) or something along those lines.
Yeah, ideally that would be the way to go. But size and cost are really the most limiting factor.