The only thing I understand the label field to do vs a custom deal details field is apply a color to the deal tile. And I would not want multiple colors displayed on the deal tile. It would lose all its value and be way too confusing. We use the deal label field to identify what divisional/departmental service is related to the deal: Landscape construction, yellow. Landscape maintenance, Green. Snow and Ice removal, Blue. For us it's perfect and having additional labels and colors would bring no value. A person can use a multiple entry custom deal details field to accomplish something similar to tagging. Multiple colors for multiple labels being displayed on the deal tile doesn't make sense to me.
Not only for Deals but also very interesting to be applied also to Organizations.