Is every part of a virus important for replication?
Unless you have a specific virus in mind, I can speak generally about viruses. It seems that the viral genome is reduced to just essential genes. Many viruses segregate their genome into periods of expression: early, mid and late. The early proteins are so-called because they encode for proteins that help their entry into cells (what is called infection). Middle and late genes tend to help the virus co-opt cellular machinery to aid in viral replication. Late genes are then useful for final stages of viral assembly and ultimtely their exit from the cell (lysis) and the newly made viral particles can go on and infect more cells. When viewed in this manner, it follows that removing even one coding region from the viral genome would inhibit the overall infection/reproduction/assembly process. In fact, if you remove the gene encoding for a viral envelope protein, purified virus added to cells in culture are able to infect cells, but not produce complete and functional viral particles. In this sense, they are infection-competent, but reproduction-incompetent.