I do not write much outside generally. Still, I would say that hubpages do help in writing in general.
1. It gives more practice to writing which in turns helps in writing better quality at faster rates.
2. While it forces one to research at times before writing, it allows one's knowledge to increase which ultimately helps in writing on similar topics from different perspectives, whether on hubpages or elsewhere.
3. While writing on a specific subjects, other ideas do crop up. So writing helps generate ideas that could be developed into articles on hubpages itself or outside.