to get your cisco certification and work ONLY on network side of IT then NO you do not need to know this. But it is recommended so that you will have a more rounded resume to get a higher paying job and to keep that higher paying job if cuts are in the works later on. You will learn command line because that is the heart of cisco switches but it is the cisco IOS operating system you will be working with, not DOS.
as far as does the cisco tech need to know how to fix the servers if they crash, well that will depend on how large of an organization you work for. If the organization is large enough that they have dedicated network only personnell then no. But you will need that experience to get in the field and work at smaller organizations that cannot afford the speciality of a network only person before you can move in with the big corporations.