Cucumber thoughts

A benefit of test driving Rails applications, especially underlying
 model behaviors, is the documentation that results from test driving.
 When I want to quickly ramp up on a new Gem, I always look at the
 test suite for insight into how the Gem or subsystem works.

I've been diving into (http://cukes.info/, "Cucumber") recently
 and have been a long time user of (http://fitnesse.org/ "FitNesse"),
 although with Cucumber I've recently watched  developers opt to not
 write unit tests and replace their test suite entirely with Cucumber features.

Even with FitNesse I was still driving my test suite from individual
 units and using FitNesse for true acceptance or classic black-box tests.

The question quickly becomes, how do Cucumber tests help better
 document developer intentions?

In addition, how do I extract common, re-useable modules into Gems
 or Concernes from an application and include them within another
 and maintaining confidence in my test suite. Do I attempt to pull
 the associate Features as well?