Multiple processors should only be introduced if there is a need to do more than one thing at the same time, not just to improve speed.
Yes that's true, especially given the range of fast CPUs these days, multi-CPUs is a pet hobby horse of mine and even I struggle to justify it and for simple speed increase I can't at all.
In areas like offloading time-consuming IO (like the Lynxmotion SSC-32 Servo Controller) or maybe handling a complex serial protocol (technically the W5100 or FT232 are co-processors) I think there's a place for them.
Rob