If you are familiar with SolidWorks, I think what some users here are requesting is called "parametric behavior".

The regulars on the Xara X forums are famous for their work-arounds, but this is the Xara LX forum! This is where I hope we expand/streamline Xara, not work-around.

What lacks here is the ability to deform a rectangle with rounded corners, and still be able to retain the fillet radius, or even modify the fillet radius after the deformation!

SolidWorks is a master at this, but I would only recommend seeking inspiration from it for the feature set, because there is not much to be gained from looking at the UI implementation.

With this said, I'd like to side with the users who wants an option to prevent the deformation of the fillet (rounded corners) when the rectangle is deformed.

Actually, it is the same with rotations of simple objects, and I have mentioned this before: Rotate anything by X degrees and after the next transformation/deformation, the X value is lost forever. It shouldn't be. It should be parametric so you can keep track of your modifications! (Same with scale, for that matter...)