Everyone has a different opinion on this argument. For me I am against deleting a hub, especially if it has been around for six months or more simply because the age of the URL provides extra weight fir search engines. Even if you give it a spring clean and then use it for backlink value to another hub or outside blog or site you will at least get something back for your work.
As far as having an account and writing on a single topic.
Thats fine if you take this road from the outset. But by the very nature of hubpages being a virtual sandbox for writing and internet marketing where you can experiment with different writing styles it seems to be a bit of a waste to restrict yourself to a single topic unless your absolutely sure you want to specialize on a specific niche or subject.