You are probably right, that two unhealthy specimens can bring some bad genes to the table. Or to the dog uterus.
However, there is a freshening of the lines that two divergent dog breeds mixed will bring. And one probably does want to keep some positive known characteristics, or have hope for them (cuteness, hunting ability, size, personality..).
Dog genetics is a very complex thing.
You can try and improve one thing and wind up making something else exponentially worse.
It sucks.
:(