Well, some of the most heavily advertised bees are "mongrelized". Such as Buckfast, minnesota hygenic, two that come to mind.
Breed for traits. Unless you have really strict breeding regiment for the bees, you won't end up with inbreeding. Besides, why would anybody care that they have "purebred italians" as long as the bees are great performers?
Look at dogs...they breed for certain traits, but only within race. They end up with health problems.