I have a mixture of on-site and off-site blogs, which were all set up in the last 9 months. Time and time again I've seen upward SERP movement for keywords recently targeted in on-site blog posts, but I haven't seen comparable shifts for sites which just have an off-site blog.
It really annoys me when off-site blogs attract organic links which I would prefer went to an on-site blog. That said, I can't quantify whether it is more appealing to link to a blog which refers traffic to an eCommerce site rather than to a blog on an eCommerce site - I think it would influence the choice to give a link in some cases, but I can't prove it.
We may find over time that the off-site blogs attract enough links to pass decent link juice. We do know from customer feedback that some people are buying from us rather than competitors specifically because they found the blogs helpful, even when the blogs were off-site only.
Although I lean towards predominantly using on-site blogs, I think there is a place in a marketing campaign for having a presence in other places whether through external blogs or careful article placement.