Between the three of us, we often debate the benefit of so-called "Recovery Runs". In those debates, I am most often arguing FOR recovery runs. I have to admit, however, there isn't much that heals the body as well as good old rest. Still, I do recovery runs multiple times per week instead of simply taking those days off. I'm not going to debate whether they promote blood flow, flush your legs of the waste created by harder efforts and so on and so forth. I'll leave that to the magazines and bloggers with a more scientific background. Recovery runs serve a different purpose for me, however.
The primary reason I'll hop on the treadmill for a nice and easy 3 miler, or slowly run a short, flat loop near my house after a tough workout is so that I feel better going into the day after that. Feeling better on that day means I get in a more productive workout. I can't say my legs are "healed", but taking a day off between one hard workout and another hard workout isn't the recipe to success for me on that second workout. If I were to take that day in between completely off, I would stumble through the first few miles of the run before I finally got my legs under me. By doing a recovery run on that in-between day, I tend to start off feeling much more fresh and ready to get into it.
This approach, of course, still takes into account the fact that I need days off. I hope that I can get to the point where I'm able to run every day and be no worse for the wear, but I'm certainly not there yet. I will generally take a day off after an "Easy Run". Easy runs are a whole other beast that I think is very well covered in the linked article over at The Logic of Long Distance.
I think understanding easy runs is a big part to understanding how to approach a recovery run. An easy run is neither too fast nor too slow. It's very much like Goldilock's favorite temperature porridge, JUUUST right (for the day, for your physical state on that day; perceived effort vs. actual effort). A recovery run, in my opinion, cannot be too slow. Whereas on an easy run I would recommend hiding your watch and not thinking about your splits, rather, listening to your body and judging your effort based on how you feel, with a recovery run, i think you actively need to keep your run at a pace you consider SLOW. For me, at this point in my training, I tend to run recovery miles anywhere between 9:00 and 10:00 per mile pace, depending on the conditions I am running on/in and how beat up I feel on that day.