There is no doubt that an aggressive back linking strategy is key. In addition to having the inate possibility for high SERPs with Wordpress.com, quality content and links to that externatl site are key as well, so I don't disagree with you on this. I use wordprees.org (standalone) site for myself and recommend it to my clients because google still loves it and because of the many amazing plugins available for it, but using wordpress.com in addition just makes sense in a well rounded IM plan.