Uncovered Raw Meat or Milk in Fridge
There is no practical difference in spoilage time for wrapped versus unwrapped.  Spoilage is a factor pretty much of temperature, since in practice, all foods have pathogens present which can breed.

Assuming your refrigerator is free of insects, dogs and similar macro-fauna, wrapping is to prevent odors from going from one food to another, drying, or cross-contamination of one food by another through drips or splashing.  Now, outside of a refrigerator, where ants, flies, and so on are abundant, wrapping also provides a physical barrier to prevent infestation, but the body of the refrigerator already does that.

In fact, in some rare occasions, you may wish to specifically refrigerate your meat unwrapped:  for example, after brining a turkey, refrigerating it for 24 hours unwrapped will permit the skin to dry out, and permit a more crispy result.