The use of shared server is to effectively share the server processes among for doing the work. The number of server processes allocated in comparison to the count of teh user processes is very less. In this case, if you would allocate a job which is going to block server processes for a long time, it won't be really good for the overall performance. That's the reason, time consuming operations like RMAN backup jobs etc are not allowed via a shared server connection but need a dedicated connection. The other attributes remain as what they are in a dedicated connection.
More generally, following the Documentation, when you need Administrator Privileges (this is the case when you Startup/Shutdown an instance or perform a Recovery) you must use a Dedicated Server Process:
Hope this help.