The coating is on there in case the pills are handled by a female. Proscar can cause birth defects if a pregant woman handles it. The package insert warns about this. The coating is not there to protect the ingredients:
"Proscar is not indicated for use in children or women. Proscar is contraindicated in patients who are hypersensitive to any components of this medication and in women when they are or may potentially be pregnant because it may cause abnormalities to the male baby's sex organs. Women should not handle crushed or broken tablets when they are pregnant or may potentially be pregnant. Tablets of Proscar have a coating that will prevent contact with the active ingredient during normal handling, provided that the tablets have not been broken or crushed."