It can be important to provide a summary for your hub for a couple reasons. First, when you publish a new hub it will appear on the front page of all those hubbers that follow you. If you've provided that summary, it will appear there. If not, it will use the first few lines from the hub itself which may not be as desirable. At least, I think that is the case.
Secondly, a good summary with appropriate keywords will help your hub be found in Google searches as it is used to populate the content metatag.