Some of my older hubs which are not drawing traffic I will go in and tweak. If that doesn't work they are unpublished and replaced with something else. I am in and out of hubs lately on a daily basis. I have to read most again to comment so if I find a way to improve the summary I do.
Time is precious so use it wisely. What I find increases traffic the most is the sharing many hubbers are involved in. Sharing has actually done more for raising search engine views than changing words in hubs. Rewriting summaries is where I would spend any time with older hubs. We share very few with our hub pages followers. We share with twitter, facebook, linkedin, a social site we publish on, and StumbledUpon.