In the context specified by the OP, 是 is not desired. In other contexts, 是 may be needed, for instance,
B: 这条裤子是太大了, 不過既然買了, 總得有人穿.
是 in the above sentence can be translated as "indeed" as a confirmation to the other speaker's statement/comment.
As a more elaborate phrasing, then the function of 是 in this type of sentences can be said to highlight the following word for a number of communicative purposes: contrast, contradiction, confirmation, etc.