For my organization, fetching and building Carthage dependencies is currently the longest part of our CircleCI integrations.
Thoughtbot has come up with a solution that they detailed in this blog post: https://robots.thoughtbot.com/caching-carthage-con-circleci
This is an acceptable solution, and we will be implementing it. However, we have a number of internal libraries on CircleCI, and would require duplication of configuration across all of these repositories.
If Carthage is used by many iOS/OS X developers on CircleCI, caching those dependencies and only updating them when necessary could result in time and cost savings for everyone.