I agree with the other posters. Security and flexibility are key factors.

One more I'd consider is portablity. I had stores with Monster prior to the buyout. My experience wasn't the best with them and moving carts was a HUGE pain in the but. You can lose ranking and need a PHD to get the 301s right to prevent it. If I was using an opensource cart, I could have just easily packed up and moved to a different host. This is a whole lot easier than switching solutions.

Here is a blog on this very topic:

Hosted Shopping Cart or DIY Cart? | eCommerce Shopping Cart Solution Reviews

I'm also of the school of thought that merchants should focus on their stores and not have to be sys admins. You can do both but it will take away from your time to do other - more important things, e.g., SEO and cust service.