You do not need to allocate an Email to Case Premium license to every user in your Org; however, each user that intends to interact with the Email to Case Premium application will require a license.

• Each user that responds to cases using the New Comment page will require a license
  • You can purchase 50 licenses, for example, and identify 50 specific Salesforce users that get them.
  • Sales & Marketing Users, for example, do not generally need licenses.
• External users sending emails into Salesforce do not require a license
• Each internal user that responds to cases using the Email Distribution feature will require a license
• Each internal user that needs to see or interact with Case Presentation Layer will require a license
• A single license is required for the "context user" that you specify during initial setup
  • The context user is ideally a dedicated API user and can be changed by navigating to Setup > Develop > Email Services > Email to Case Premium and editing the routing address(es) listed.
  • If you do not have a dedicated API user, it is generally best to use (and license) the user you specified as the Automated Case User under Setup > Customize > Cases > Support Settings.
• Self-Service, Customer Portal, and Partner Portal users do not need licenses because they don't interact with Email to Case Premium directly
• A license is required to configure Email to Case Premium

For more information, see "Assigning Licenses for Installed Packages" by clicking on Help & Training at the top of your Salesforce.com account.