Good question.

Because the whole point of running these sites on the test machine is to run tests. And one thing that I want is the code coverage of those tests tracked. The DotCover code coverage tool only retrieves the code coverage when IIS Express closes down gracefully. If I use /F, I get no code coverage.

Put it another way: we were already using /F but getting no code coverage results. Now I want code coverage results.

I don't know what makes the sites close gracefully on my local machine, but refuse to do so when running the test suite on team city.