Are mixed breeds genetically healthier than pure breeds?

But Mark - I'm going to use puppy Ben as an example:

Aussies have their problem with certain chemicals (somewhat herding-group specific)
Poodles have a high incidence of patellar luxation.

By crossing the two - did I lessen my chances of having a chemical-sensitive dog who's slightly less prone to patellar luxation?? Or am I whistling through the cemetery? ;-)