The main idea behind this property being set to false, is that the control should not unnecessarily modify the developer's logic. Actually, in most scenarios, there is no need to enable this property.
As for the problem with setting this property to the manager, not to the window, I don't know what your exact setup is or how you open that window, so I cannot tell why the property didn't work when you set it to the RadWindow. If want (and if you have time of course), you could send me a sample project where this issue can be reproduced and I will check it.
the Telerik team