A cracked Crock-Pot insert can still be used if the insert is ceramic. If the insert has a non-stick coating, it should be replaced and not used.
If leaks are a concern when using a cracked Crock-Pot insert, both Crock-Pot and Reynold's manufacture slow cooker liners. These plastic liners are placed over the insert to make cleanup easier, but the liners can also be used to catch spills when using a Crock-Pot with a cracked insert.
If the crack is rather large, it may be best to replace the insert. Crock-Pot has several replacement inserts and other parts for their products available on its website.