This hasn't come up, but it will eventually.
Let's say you have a business zone that allows medium intensity retail uses. A person comes in with a proposed use that seems marginally too intense to you based on the interpretation of the ordinance, but you can find examples of similar uses existing in other places where that zone applies.
You have to make a decision on whether or not the applicant's proposal fits in the zone. In your decision, which would you consider? The strict interpretation of the ordinance or the precidence of the existing businesses in the same district? If you judge that it isn't allowed, did you just create a bunch of legal non-conforming uses in your community?