It should definitely be covered-up to protect it from becoming a toilet for the local cats, but not with plastic.

Better to cover it with a sun shade type material that allows aeration.
covering the soil will stop it from becoming dry and blowing around.

If the pile is to be in contact with the ground, this will help in retaining microbial activity in the soil. Meaning it should remain fertile and ready to use when you are ready to use it.