Under Google's Caffeine search ranking algorithm, page views and user engagement has a high factor in affecting search ranking for a site or page.
Long blog posts / pages on a single page output generate no user engagement factor under caffeine other than the length of time a reader stays on the screen. Splitting the post / page across several pages accelerates the user engagement score due to them navigating through the site.
In earlier incarnations of WP, the nextpage button was on the visual editor next to the button for the more tag. It's been on holiday a long time and never returned.
Manually entering the nextpage tag in the visual side of the visual editor does not work, you have to go to the html editor, add it there, then return to the visual editor - this is a pain and annoyance for those of us who are verbose in our writing, and with readers who prefer in-depth articles rather than extended-tweet style blogging.
Please return the nextpage insert button to the visual editor - ideally as soon as WP 3.1 - it should be a simple matter to do.