should door locks and deadlocks be keyed the same
The big disadvantage is having to keep two keys with you instead of just one.  If you lose your keys (or someone steals them) both keys are compromised so there's not an advantage to having two different keys in that case.

The only advantage I can see is if you frequently need to let other people into your house.  You could hide the door knob key or give it to the third party, and on the day when they need access leave the deadbolt unlocked.  That way the door is still locked and the third party only has access when you allow them to.  Any other time the deadbolt will be locked so even if someone finds the hidden key they can't use it.

It's a bit contrived, but using the two keys as a "double lockout" type scenario is the only advantage I can come up with.