By having mirrored proteins and sugars, such organisms would have no predators or parasites. This condition -- having no natural predators and parasites -- has been replicated many times through the dislocation of species through human transport. Often the result is that such a species is at a huge advantage, much to the detriment of the environment. These are called "Invasive Species." (Asian Carp, anyone? Brown Snakes in Hawaii? Kudzu? Rabbits in Australia?)

Mirrored proteins and sugars would be much worse, since it would be exceedingly hard to evolve an ability to decompose these. I imagine that would happen eventually, but it would take such a long time, our environment would be in shambles and we'd be all long dead.