1) No, Desktops don't need it. It's plugged in all the time, whereas Optimus leverages the iGP to save battery when not plugged in. Desktop GPUs just downclock themselves to consume less power in this state to put less stress on the GPU.
2) If the motherboard supports it, for Crossfire yes. Nvidia does not support SLI with Intel integrated graphics, but I believe AMD supports Hybrid Crossfire with onboard-APU graphics on their FM2+ socket.
3) I have no idea what QuickSync is or how it works. But most current generation intel and AMD boards do have a VGA, HDMI, or DVI slot for the onboard iGPU.