Conclusion
It is important to understand the different visitor types and ensure your site caters to each one. You need to understand how the different conversion factors are combining on your site for each of the visit types. The math used to put these considerations together can be as simple or as complex as you like, but you need to do the work.
My thinking on this topic has been heavily influenced by the paper "Which Visits Lead to Purchases?" written in 2000 by two researchers in online behavior, Wendy W. Moe and Peter S. Fader.
If you want the full mathematics for such analysis, their paper is a good place to start.
