Stalling roof turbines to improve air flow?
Wow, landlords can really be dummies sometime.  The simplest answer is that the turbine company paid an engineer to design/improve their turbine to be most effective relative to the cost of production.  If stopping the blades were more effective then there would be no bearings for them to spin on and the turbines would cost less to build.  The landlords idea could be improved by taking the turbine off the base and leaving the attic open to the sky.  This, however would allow rain into the attic just as what would happen with a driving rain with stopped blades.  Empirevent.com has a nice website with photos.  You can easily see into the interior when the vent is stopped and that is the path for rain to enter.