I'd suggest its bad form to delete comments that add context to an answer -- edit that information into the answer first! (Obviously preferable for the person leaving the comment to use the edit button, but the world is seldom perfect.)
This is especially true since there's no notification that comments have been deleted.
Or in other words:
(Self-contained answers) > (Incomplete answers with additional information in the comments) > (Incomplete answers with no information in the comments)