I find it cumbersome to make sure all test results related to a Jira Issue are linked to the Jira Issue, and besides, if I do, I can only see the results of the test-cases that actually have a test result. If I want to see the test-cases that has not been run yet, I need to link the test cases to, not just the results. It's easy to forget a link or two and because of this I find it hard to trust that the test status I see is the correct one.
So, I would have loved to see a package of three new features within the TestRail - Jira integration
1) The ability to link a Jira Issue to a Test Plan or Test Run in Test Rail (preferably a "Create Test Plan" button in Jira that creates a ready linked Test Plan or Test Run in TestRail
2) The ability to click on the Test Plan / Test Run link in Jira to go to the Test Plan / Test Run in TestRail
3) In stead of displaying all test results in the Jira Issue, just display a piechart or some kind of a progress bar that indicates how many out of the total number of test cases cases within the test plan / Test Run is passed, blocked, failed, not run etc.
By implementing this you get a way more lean and powerful requirement coverage capability.