In Service Desk there are Agents (licensed) and Customer Portal users (unlimited, yet required to create account). Developer is external from this point of view, and can usually interact with the project internally, with no customer interaction possible. It is Agent who works as FLOS and dispatches requests to developers.
It's covered pretty clearly at https://confluence.atlassian.com/servicedeskserver034/setting-up-service-desk-users-875313790.html
To answer your question: end user does not need Agent license, and Service Desk license does not limit end users, custoemrs and developers access toproject in any way.
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