When you go to dump, having separate tanks lets you dump the black tank first, then use the grey water to flush any remaining nasty stuff out of the hose before you disconnect it. If the tanks were combined you'd have to rinse the hose much more thoroughly by hand.
Overfill a grey water tank and grey water will backup into your shower. Connect your shower (the lowest drain) to a combined black/grey water tank and guess what you'll have to clean up.
If you're boondocking for a while, it's easy to get a cap with a garden hose fitting, connect a short piece of garden hose to it and drain off 5 gallons of greywater at a time into a portable container. Then take that container to a dump station. While you're there, get 5 gallons of freshwater (in a second, clean container) and bring it back to the rig. Doing this daily, I find I can easily double the length of time until I run out of water and/or holding tank capacity.