Originally Posted by jayeff

Even if we were discussing one inbound link on one site, there isn't an automatic benefit based solely on relative PR. And although from the very narrow perspective posed in your question, such a link would likely be more good than bad, any advantage would be very small: only one of many factors which determine the site's SE placings.

In the real world, what we see more commonly are lists which sometimes run to dozens of reciprocal links. Even if everyone involved has taken care over their choice of links (unlikely since most such exchanges happen when someone promotes them), the biggest flaw in non-natural linking is that it can easily be identified as such. When it is so identified, about the best you can hope is that you aren't actually penalized.