You've got to remember that all pure breed dogs at one time were a mixture of different breeds to perfect the standard. Take the American breed, the Chesapeake Bay Retriever - It's a Newfoundsland and Lab mix in its ancestory... Thru careful breeding we now have the standard Chessie today.
Some mixes are accidents - like my Shadow - Lab/Boxer - thrown away with his sister by time he was 5 wks old. Others are planned to perfect a new breed for the future. Just do some research and read about the different breeds and their backgrounds... They were "mutts" at one time too... How do you think some of these breeds came to be? They were created by man trying to figure out what they wanted for a dog.
Ellen and Shadow Walker