If you are striking the sharp edges of cutting tools a lot, the plastic tip may wear or chip away. You will want to replace it if the plastic is worn away enough that you may start making contact with the aluminum body of the hub.
Think of a pencil eraser. You can still use it as it wears away until the metal sleeve comes in contact with the paper.
Hammer tips are kept in stock, part number 084B04 (MSC #16432973)