When you get a PnP, people start asking if you do contract assembly. I always say no because I'm already busy with assembling my own boards. As designers, we pretty much want don't want to spend all of our time on assembly which is one of the reasons for having a pick and place. However there are some interesting possibilities with having a PnP. If you already have a storefront, then one possibility which is used pretty well by Sparkfun is that you can provide a service where you sell other people's designs in your store but you also assemble them. Its interesting because it addresses needs on both sides. On one side, sellers usually end up spending a huge amount of time on hand assembly and so you see a lot of people come out with a couple of PCBs, and then drop off the face of the planet because they determine that selling boards is much more work than it looks like. On the other side, people that buy a pick and place machine are looking for different ways to keep it occupied to pay it off. The majority of the components, as LadyAda mentioned, are jellybeans which are very cheap. The labor required to mount them by hand usually far exceeds the cost of the components. Ideally, if you combine the two, the designer of a board can just hand off the PCB to the pick and place owner with the BOM list. The PnP owner can manufacture a couple of the boards and sell them in their shop. The designer would get a royalty (or something like that) and the seller would get a margin that reflects the parts sourcing, assembly, and distribution. Both sides end up getting paid and the PnP is leveraged to carry the burden of assembly (except for the setup and mounting the specialized components).
I suspect that its pretty labor intensive though which is why I won't be doing this anytime soon. It is pretty interesting though and would be an ideal model for micro-manufacturing, or how to efficiently make products targeted at niches.

