My thanks to user320, who found this Salesforce.com Article for me to reference.
Salesforce licenses are designed for users who require full access to standard CRM and Force.com AppExchange apps. CRM apps are anything that requires access to Standard Objects like...
Salesforce Platform licenses are designed for users who only need access to custom apps, and NOT the standard CRM functionality. Salesforce Platform users DO have access to the "core" Salesforce Standard Objects and functionality, like...
- Custom Tabs
So, if a user only needs to access custom Force.com or AppExchange apps, it's OK to assign them a Salesforce Platform license. On the other hand, if a user needs access to any CRM functionality (eg Sales or Service Cloud), they must be assigned a Salesforce license.