Liabilities of Platform Ecom
I know it is strange. I am normally PRO platform (See Platforms vs. Websites http://scenttrail.blogspot.com/2011/09/internet-marketing-platforms-vs.html#.UOwaJonjkeY ). 

I am NOT pro ecom websites using an aggregator such as Volusion or Shopify except as a snapstore supplement. In this equation Shopify or Volusion is the platform NOT people creating stores there. 

This means the platform benefits accrue to Volusion and Shopify. It is not wonder I can't fid a store inside the Volusion ecosystem that has higher PageRank than the loft 7 Volusion puts up. When you see a platform pulling a PR7 or 8 you know that SEO benefits are flowing to the platform and less so to those using it.  

It doesn't mean you should use aggregators, it just means don't put your single store there. If this sounds like we are returning to the need for diversification you are correct and win a cookie. The problem is more than one store can be a lot of work. 

When we added a new store when I was a Director of Ecommerce it tripled the work. If your IT is up to a feed based system where you maintain a core database once and it feeds all other stores via web services then consider a blanket or tapestry of stores. If your IT looks at you like a deer in the highlights when you ask that last sentence then you better stick with one store just make sure it is YOUR store not THEIRS (i.e. on OPP or Other People's Platform).