The FishEye Linkers functionality applies to commit messages only. But commit messages are often contain the same references to Crucible review descriptions and comments: e.g. developers refer to the same issue/bug ID in their bug tracker, and Crucible review descriptions are so many times exactly the same to commit messages. So it would greatly improve the review workflow if FishEye Linkers would work in Crucible text fields instead of having developers to use wiki markup for a functionality that works out-of-the-box in commit messages.
Also refer to this thread for the original idea: https://answers.atlassian.com/questions/32527955/fisheye-linkers-wont-work-in-crucible-comments