When Cobb first meets Yusuf in Inception and tells him he is seeking for a chemist to formulate a compound for the Fischer job, he says that he would also need to "go into the field" with them to tailor compounds specific to their needs.
However, once a group starts their shared dream, they cannot wake up until the time for the dream is over, especially if they are sedated. Yusuf will not be able to change anything about the sedative once the dream is in progress.
So why does Yusuf need to join them into the dream? Isn't it enough for him to tailor a sedative suitable for a 3 level dream and hand it to them?