We need a way to know which Test Cases are covered by Testers and which have yet to be assigned. 

Because we have so many Test Cases, we assign them in the bundled Test Sets instead of going down one by one and assigning them by Test Case. However, by doing it this way the Test Cases don't fill in an owner (your terms for tester). When we go back to add Test Cases to another Test Set we can't tell which are and which aren't already in a Test Set and assigned to a Tester.