True.
Sometimes, if breeders are useless.
Total myth.
Slapping two different breeds together in no way guarantees that only the best genes make it through.
It is a complete crapshoot, just like how 2 human parents have no idea which traits/genes they will pass down to their children.
Keep in mind, things are no longer doing the 'natural' survival of the fittest, which would over time cause useful traits for survival to become dominant, now we are interfering an creating many breeds that would never survive without human intervention (pug anyone?).
Part of the problem of these cross-breeds is people just think it will automatically pick the bits of the breeds that they want. People usually get a specific breed because of its traits. If the dog is a mixed breed it'll exhibit different traits. For example, if your Lab is mixed with Collie it'll likely be very nippy with a predisposition to herding. It may also be neurotic and to top it off, it may have some fatal genetic issues that you don't know about - like the MDR1 gene mutation that collies have.
Different breeds have different needs and different developmental stages. A Collie in a Labrador body is likely to hurt itself because it's definitely not as agile as a Collie in a Collie body.