One of the reasons people use dry rock is to control the amount of hitchhikers coming into the system. With regular live rock you have zero control.
If you purchase 'cured' wet rock you could get any number of undesirables which some like and others do not. Like bristle worms, crabs, asterina seastars and microalgae to name a few.
With live sand there is a lot of debate in MR with most reefers saying it is not a needed expense. The main reason is you can use dry sand and add some sand from an established tank to create a biological component.
Dry rock is less expensive too. all u need to make it "live" again is add a small piece of live rock from an established tank. same with the sand. http://reefrocks.net/