They don't lower the internal temperature of the freezer. The frost generally forms first on the evaporator unit and that is the part that is heated periodically to remove the ice buildup. It's possible for ice to build up elsewhere in the freezer but the air is circulated and the remaining ice generally sublimates away. This wouldn't cause your food or a bag of ice to melt, but it will cause your ice cubes to shrink over time and will freezer burn your frozen food.
In my experience, food in self-defrosting freezers doesn't outright spoil quickly enough to be a practical concern, though I'm sure it does technically allow for stuff to grow faster.
What it does do, though, is get freezer burned very quickly.
