Adding my +1 for adding the ability to link JIRA issues to a test plan or run.
We've just this week been able to add in the more full JIRA integration, and so far am loving it! Except for this one wee gap.
A little more from my perspective.
Using the "Reference" field in test cases makes sense for us when introducing a new feature and we're adding in new test cases. Using the "Defect" field in results makes sense when we want to have a link between a failed test result, and its corresponding issue in JIRA.
But as others have noted, there are times when we want to use existing test cases to cover off the testing needed for a particular issue. Or we may want to use a mixture of new and existing test cases. In some cases, there may be many existing test cases needed.
We don't want to have to put JIRA issue numbers into every test case that might be run. Our reference fields and ties back to requirements would get all tangled up.
We don't want to have to use the defects field for anything but defects. If we did, our defect reports and analysis in TestRail would become meaningless.
One further note, I wish to truly use JIRA and TestRail in a tightly integrated way. Meaning, I want to be able to go through my JIRA issues and see that we have test notes captured in TestRail. For now, we're requiring testers to put a link to the test run or plan in the comments of the JIRA issue upon closure. We could add a text reference field for that value. But it would be super awesome if we could have information about the test run/plan pulled from TestRail and displayed in JIRA, rather than having to click on a link.