Why is subclassing not allowed for many of the SWT Controls?

hi guys,

Why is subclassing not allowed for many of the SWT Controls?

But Eclipse itself creates our Shell classes in this way (inheriting from Shell) and then overriding checkSubclass() with empty body. So is it good(safe) that we do this way? I have two shells (Shell_1, Shell_2). Shell_1 is parent of Shell_2. I only want to extend from Shell class for Shell_2 so I can simply add controls on it inside the constructor of Shell_2. Is this wrong way to do?