is EJB,s knowledge Must?

What problem does developer has to face when he has no idea about EJB but he is aware of struts and Spring?

Are you talking about EJB 2 or 3? I still see some legacy applications using EJB 2, including IBM products like WebSphere Commerce. EJB 3 is seeing some adoption, but many of the applications I use just leverage JPA without EJB 3. I still recommend understanding the core concepts of EJBs as they are useful patterns. This includes understanding the difference between entity and session beans, transaction management, etc.