We're working to position Ninite as a great general download site and our new Ninite updater should help repeat use. Even with a single app, clicking next next next through an installer is silly. Our paid users end up using Ninite daily at their work already.
A big issue with SEO for us is that normal users don't even imagine that what Ninite does is possible. I think some mix of education and oblique SEO could help, but I think the newness of what we do complicates things a lot.
We aren't hurting for traffic, but it's primarily word of mouth and press so far.