Radio buttons cannot have the value false; because they check for a falsy value instead of an undefined value, they never return the correct value when queried (eg with #getGroupValue.)
That is, if the declared value of a radio button is false, the radio button will respond with nil when queried for its value. The effect (and visible indicator of the bug) is that, when a form is displayed and the value of the record's corresponding attribute is false, the checkbox with value=false will never be checked when you open the form.
In my app, we have a boolean field, "is_private". But we want to display the field to the user as a set of radio buttons, one with the label "Public" and the value false, and the other labelled "Private" and the value true. With this bug, the form would never display a false value for the attribute. Even if 'private' is false, neither radio button would be checked when opening the form.
Interestingly, this same line in the getValue() function has been bug-reported and fixed once before. Previously it did not work with any values. The fix made it work with all values except booleans: http://www.sencha.com/forum/showthread.php?200708