G-D created different races. They each have certain strong points. Germans and Japanese are good engineers, The English are good sea-farers, the Italians create beautiful shapes etc. Do we wish to effectively do away with all these attributes? I know many mixed race people who are very nice but the question of whether it (race mixing) should be encouraged remains.
Consider this:
Man has created different dog breeds. Newfoundlands are excellent swimmers and save many humans from drowning. Blood hounds can track escaped convicts over any terraine. Border Collies, Kelpies and Cattle dogs do the work of several men when working livestock. Fox terriers have no equal as rat killers.
Mixed breed dogs are very nice, but imagine if we just mixed all the dog breeds into one breed of mixed-mutt. We'd have none of the specialist quallities of the above pure-breds.