I view mixing BP morphs as being purely aesthetic - the snake is still a ball python, it just has different coloring/patterns. It still acts like a BP, with all the same general temperament and habits, etc.
But dogs are not BPs. When you mix dog breeds you warp the fundamental aspects of the dog; its intelligence, temperament, behaviour, not to mention the various physical traits. Mutts really are mixed-bags, no pun intended.
When you have a pure-blood dog you can safely make statements like, "such-and-such is typical for this breed." With mutts you can't do that. One may inherit the loyalty of its fathers breed with the hunting instinct of its mothers, while a sibling may inherit a warped hunting streak from mom resulting in heightened aggression, with a warped sense of loyalty to the person it views as the 'pack leader' to the exclusion of all others... including your children. You just don't know.
On an ethical note; mutts also inherit the health problems from both of its parent’s breeds. Keep in mind that often the breeds being mixed originate from radically different parts of the world, where they’ve respectively evolved to possess traits specific to that location. Was a stout thick-furred Husky really meant to breed with an Arabian racing hound? Its offspring (I think they call it a “Huscbian”) suffers from crippling arthritis and is prone to heatstroke, seizures, and a plethora of other medical issues which many feel is a result of the mixing of such opposite breeds. But people still do it. They look "cute" as puppies.
I am against this new craze of creating mutts and calling them "designer". I think it dilutes the gene pool to a detrimental end.
There are people who've spent centuries maintaining breeds of dog with pure blood for a reason. Sadly, I think the vast majority of people who take up this latest craze are motivated by one thing... $$$.