100% true, but trying to build external contents (more so if it's not written properly) just to create links can seriously affect a website's SEO ranking (resulting from Google penalizing it).! Also remember getting links from sub-domains are not that much useful. Also recently most SEOs had given up spending time on sites like Wordpress, Tumblr and so on, were you only get a link from a Sub-domain...!
However if you are sure about developing quality contents, then you may press on work your way up the SEs.!