Yes, it can help. It's not going to necessarily skyrocket all your search rankings but link juice can flow over from subdomains. Not as much as it once did... but still somewhat.
Yes, a good internal linking structure contribute a lot in improving your web rankings. In fact its been confirmed by Matt Cutts, http://www.keywordcountry.com/blog/...-reveals-4-killer-seo-tips-to-his-kidnappers/ A good linking structure affects the overall reputation of the website.