The real question is, do you have the time, energy and resources to manage more than one website / social media / seo campaign really well? Having worked with companies who tried that approach, it seemed like they split there attention and none of them really panned out. The ones that got the focus did well. I'm not saying you should not do the subsidiaries, but perhaps that can come later, if one area grows beyond what the site can do from a content standpoint, or needs more SEO attention. As Alan mentioned it's about the brand, and in my experience, managing one brand vs eight is more resource/time/cost effective.
