Can the MTM Test Run 'build number' field be unhooked from a real build without using a fake or empty string.

Question

I'm working on integrating MTM Test Runs (Via TFS/MTM API) with our automated build & test system, and I've noticed that if I supply a valid build number when creating a test run, and I later delete the build, the test run data disappears with it.
I know that I can deselect 'Test Results' in the Delete Build dialog, and in the Retention Policy for the Build Definition, but in the interest of avoiding any confusion or mistakes (we have a lot of build definitions to maintain), I wonder if it would be
possible to instruct the test run to prevent a deliberate link.
While we're on the subject, if anyone could explain the benefits or potential uses this link might provide (even under conditions that may not meet our usage) I would be very grateful.

Answer 1

We don't use MTM to run tests, we have another system that works better with our test environments. We will be using MTM to track pass/fail data for the tests that have run, by creating test case workitems for them and using test run and test result objects
to store the data.
Going back to my question, I'm looking for a way to prevent a test run object from linking to a TFS Build so I can prevent TFS from removing the test run when I delete it. We have a lot of builds and we frequently remove old ones, I need a way to retain
historical data for test runs.
I understand that under normal circumstances, the build number would first be used to retrieve the bits from the build to test with, but since we're not using that system, I'd like to know more about how it might be used in other ways - for example, for
querying or associating work items. We're considering not using this field, or modifying it in such a way that it doesn't create the relationship with a build that I mentioned, and I would like to know what functionality we would lose by doing this.