When designing your object model, a good knowledge of the existing Standard Objects and their features is the key to knowing when and when not to reference them. Keep in mind the following points when considering the use of Standard Objects:
From a data storage perspective, ignoring Standard Objects creates a potential data duplication and integration effort for your end users if they are already using similar Standard Objects as pre-existing Salesforce customers. Remember that adding additional custom fields to the Standard Objects via your package will not increase the data storage consumption for those objects.
From a license cost perspective, conversely, referencing some Standard Objects might cause additional license costs for your users, since not all are available to the users without additional licenses from Salesforce. Make sure that you understand the differences between Salesforce (CRM) and Salesforce Platform licenses with respect to the...