Hm. I'm not certain because of many unknowns. If your site has a lot of pageviews then it might be wise to keep them separate. The only real gain is in the upgrades to WP and the plugins - but these should only take 5-10 minutes per install whereas converting everything to a single WP install will likely take tens of hours.
The static pages I'm currently moving them to Wordpress.
I was just thinking of the ease in terms of getting Related Posts for example from the other Wordpress databases we have. If there's a plugin that can give related posts across different WP installs, I'd love to hear it!